I tend to agree with that. Though the bmx will be one fun to ride little beast, It will be a bit hard to carry a large battery all the time and have a really practical bike for 20 mile distances. More can be put in a pack for occasional longer rides though, as I said before.
Nothing wrong with the little bmx though, for the young elastic lower back and knees, and you should be able to cram 8 hardpacks into and along the top of the frame with no problems. So if 8-10 mile range is good enough most of the time, go for it. You can ride 15 mph, and go 20 miles if you must, but it will be hard to hold back and ride that slow once you have tasted 30 mph.
If at some point, you change your mind, and want to go to a 26" bike with suspension, all you will really need to buy is a spoke set and a rim, or perhaps a cheap 26" dd motor only in 26".
You'll have to spread the drops on a bmx, so get steel so you can safely bend it out, and a single speed freewheel. Once you have the freewheel on the motor, you can see what your actual spacing needs to be. You may need to adjust a spacer washer, or even cut a new axle shoulder on the motor, so you can run a spacing less than 135 mm. 135 would be for a seven speed freewheel, and your bmx won't have a derailur.
The other option is no chain, and run a 110 mm front hub on the rear of a bmx frame.
Lastly, torque arms or torque plates. You need em. If the bmx bike has a big enough steel dropout, mabye you can just do this on one side.
Edit. well one or two more last things. You are in the USA, so be sure to order your lipo from the USA warehouse if possible, for quicker delivery, and easier return if you get a stinker pack. Do NOT under any circumstances order any out of stock item from HK.
You will need some kind of charger, so EP Buddy is your buddy. You can get a reasonably good 150w 8s charger there, and a good 350 watt meanwell to power it. They also have goodies you will need if you don't get them at HK. Wire, bullet connectors, a cellog 8 for checking cells etc. Your kit will come with a CA, so once you have sorted out any potential bad packs, you can monitor the whole pack voltage good enough with the CA, and have the cellog 8 in your pocket if you need to check an individual pack or cell's voltage at some point near the end of the ride home.
If you are riding every day, charging RC lipo will be a pita with a regular RC charger. But even if you get into bulk charging , you should have at least a funky 50 w RC charger in the house. Sooo handy for balancing a wonky pack when you need to do that. Doing bulk charging right is a bit pricy, you need some fairly expensive HVC LVC boards. If you go to bulk charging, you might want to go to 6s packs, so you need only 2 boards for 12s, rather than 4 for 16s.
If what you want is basic transportation that is legal in your state, ( 30 mph is likely to be illegal) you really should be building totally different. HS motor, 20 amps, 48v 15 ah pingbattery, 26" MTB and you are not going to look so much like the illegal homemade, unregistered motorcycle that the bmx build is. About 25 mph top speed, and blending in with regular bikes a lot more. 36v, to really be 20 mph legal, if that is the law where you live.
You gonna look like a clown when you spot a cop at 33 mph, and try to act like you are pedaling that bike.
THE LIPO RULES. NEVER ABOVE 4.3V NEVER BELOW 2.7V DON'T PUNCTURE
Ideal charging /discharging range for Lipo, 3.65v minimum 4.1v maximum
See battery technology section, FAQ thread at the top of the page for lipo noob info.