So you want to build an electric bike...have you lost your mind? It is a lot of work and is going to cost a lot of money. The fully assembled bikes like the ones that Luna Cycles sells and the Radrover are something you should consider before you read any further.
Haven't changed your mind...well my recommendation is a steel frame because it is stronger than aluminum and steel will bend before it breaks more than aluminum so it gives you a visual warning that it is about to let go. Carbon...forget it, one second it is fine and the next it is broken, strong yes...up until it breaks without any warning whatsoever. The drawback, a steel frame can be a little heavier.
Instead of suggesting what to buy, I'll tell you what I did...I was in your shoes a about a year ago and I couldn't find the information below.
I spent about a gazillion hours scouring the internet and researching every bike in the world. I came to the conclusion that a Mongoose Terrex was the way to go. Now I'll agree that most Big Box Store (Walmart, Kmart, Sears, Academy, etc) bikes aren't worth taking out of the box but I found a pretty good one...the Terrex. I couldn't get any image to post so here is a link to the Walmart site with the Terex: https://www.walmart.com/ip/27-5-Mongoos ... e/45146057
More data and info on the Terrex....
I bought my second Mongoose Terrex recently and it turns out the newest ones have a 148 mm OLD. The first one I bought has a 140 mm OLD. OLD is "Over Locknut Dimension" and is what is commonly known as the "spacing". The numbers I gave are for the rear. That is the only difference I can find between the two bikes....and the rear hubs which match the frame OLD.
The first Terrex I bought has a 12T MAC hub motor. It works great on the road but I have had some issues when riding off road really hard. I ride off road about 95% of the time and climb some reasonable hills but slow speed and hill climbing at the same time are a worst case scenario for overheating. I have a 20" x 57 mm internal width rim ordered which will be laced to a MAC housing and I plan to do some experiments with ATF to see how much it helps cool the MAC. Plan on mounting a 4" wide tire on the rim...that should give me a little cushion on the rear of the Terrex since it is a hardtail. I am running a Cycle Analyst Version 3 which is commonly abbreviated CAv3....with a MAC motor that has temp sensors like the one I bought from EM3ev, the CAv3 will tell you what the temp of the motor is and roll back the power if it gets too hot to keep you from damaging anything.
A plug for Grin Technologies....these guys are the sharpest around. They sell the CA (all versions) and a battery charger called the Satiator. The Satiator is a little pricey at around $300 but you can program it to charge just about any battery and you can set it to charge just about any voltage profile you want. If you don't know already, Lithium batteries can be a little finicky and it is a challenge to keep them happy. The power output can be amazing but to make them last as long as possible you don't want to get them too hot, too cold, charge them too high or run them too low. Sort of like Goldilocks....treat them just right and they are as wonderful as a fairly tale. Grin also has a motor simulator and a charge simulator that are worth investigating.
Forks....well I had asked about upgrading forks and never got a response so I ventured out on my own. Keep in mind everything I post is my opinion so yours may be different but the best coil spring fork I could find, that fits the Terrex is the Suntour XCR. I went with a 29" XCR fork because I am running a 27.5+ tire and the 29" has the largest crown to axle distance. The XCR is also a 1 1/8" steerer tube with no taper, 135 mm OLD, a steel as opposed to aluminum steerer, and has a rebound adjustment (on some versions...choose carefully). The widest tire I can run with the 29" XCR fork is 2.8". Currently running a Maxxis DHF 27.5 x 2.8". The stock forks are only good for a paperweight....I have tried everything including disassembling them and greasing them with grease and silicone (separately) and they still stick terribley...it is called stiction, plus the springs are way too stiff for me. Why coil springs instead of air springs...I want to ride and not be checking or adjusting my forks. The coils don't change...the air shouldn't but it can be affected by temperature and leakage. Air forks are lighter and should give you higher performance but remember on the Terrex you need a 1 1/8 steerer that isn't tapered and most air forks, well most forks in general are now 1 1/2 on the bottom and 1 1/8 on the top i.e. tapered. The tapered design is better and "Problem Solver" makes an adapter where you can run the tapered forks but it gets complicated since it significantly affects the steering geometry.
The widest tire I can run on the rear of the Terrex (both OLD versions) is a 3.0"...currently running a Maxxis High Roller 27.5 x 3.0.
The second Terrex has a Bafang BBSHD and has worked nicely. I did "customize" the Freewheel (cassette) so that my chain line was close to perfect but otherwise the only change is the XCR fork and tires. I am also running a 30T Mighty Mini gear (from Luna Cycles) on the BBSHD...highly recommended if you want your motor to stay cool and operate at its most efficient rpm. Off road I run the 29 tooth or 34 tooth rear sprocket depending on how tight the trails are and almost never shift gears. The freewheel has a 24 tooth I use for getting from A to B.
I weigh 220 lbs ready to ride and I am running 13 psi front and rear. For me it is the optimum pressure to add cushion or suspension via the tires without the tires rolling over and affecting the handling. Also, any lower and when I hit a root the rim strikes the root and I'll eventually bend a rim.
Recently purchased a Mongoose Hitch Fat Tire bike and the Terrex appears to have a slightly stronger frame. The only reason I bought the Hitch was to run a wider rear tire and see if I could get enough cushion/suspension with the Fat Tire in the rear to keep my old arthritic back from hurting. I plan to run a XCR fork on it with the 27.5 x 2.8 DHF front tire because I hate how a Fat Tire bike steers, especially as you drop the pressure in the front tire. It is called "Self Steer" and the bike has a mind of its own. If that doesn't work, the next step is a full suspension and starting from scratch again...lots of options, although a Giant Reign would be my tentative starting point. The cost of any good full suspension bike is going to be more than either one of the complete set ups I am currently riding so cost is going to more than double. The biggest problem is most full suspension bikes have aluminum or carbon frames...remember our discussion earlier?
Just FYI, the cost to build a Mongoose Terrex with a MAC or a BBSHD is approximately $1,500 if you shop carefully and squeeze your pennies tightly. Roughly....Bike $250, motor kit $700, battery $500. Oh yea batteries....I am running a 14s6p 52v nominal battery. 14s means 14 batteries in series and 6p means 6 parallel strings of 14. The "s" determines the voltage i.e. how fast you can get there and the "p" correlates with the capacity i.e. how far you can go. I'd very highly recommend a 52v battery as opposed to a 48v or lower if you can afford it. You can't go over 60v with the BBSHD and the 52v fully charged is 58.8v. The higher voltage will give the motor a higher speed and you'll be able to feel the 52v is a little snappier than the 48v.
Whatever you choose....your motor, your controller, your battery, and the way you are going to use the bike need to be somewhat matched. Doesn't do any good to have a 30A controller if your battery will only supply 10A. Off road, the BBSHD is the way to go because you can use the gears to spin the motor at an efficient rpm. On the road or commuting, I'd lean towards the MAC because the power doesn't go thru your bikes chain/drivetrain...unless you live in a pretty hilly area then I'd go back to the BBSHD for the hill climbing capability. Want to go fast...you need voltage and a Direct Drive (DD) motor like the Crystalyte or the MXUS or even the Cromotor if you really want to scream. I have no need for the DD motors...they have their application but don't match the way I ride.
There are a lot of other options besides the Bafang BBSHD and the MAC but they are the two most common and cover at least 90% of the needs out there. Of those 90%, the MAC probably covers 40% and the BBSHD the other 60%. So if you do the multiplication, the MAC covers 36%, the BBSHD 54%, and other motors cover the remaining 10%...just my estimate, no specific data utilized.
Everything above is just my opinion....what do I know I am just a Mechanical Engineer with over 50 years of experience. That basically means I have screwed up enough times to know what I am doing now....sort of.
Now go back and read the first line of this post. There are a bunch of good options besides what I listed, those are just a couple good ones. If you still want to go thru the pain and suffering of building your own bike...Walmart will deliver the Terrex to your door.