Welcome to the forum
I have a MXUS 3000W V2 4T on a Townie 7D and all I do is take a flat head screw driver with the motor just about in the slot, and bend the frame out just a bit. It does not take much. I now have that same motor in a Norco City Glide which had a Nexus rear geared hub instead of a cassette and the rear dropout of that is much narrower then my 7D, but I do the exact same thing.
Be sure to use TWO Torque Arms.
The MXUS is a great motor, the Leaf 1500W motor is good too, can pump a lot of juice in there if your controller and battery allows for it.
The good thing with the Leaf is you can buy the kit, with the KT display, great for speed, distance, pedal assist modes. The Leaf is quite popular because they are cheap and they are so powerful.
If you do plan to go with the MXUS, then buy yourself a Sinewave controller from http://www.kellycontroller.com/ and be sure to get Regen because you save your brake pads.
I buy a lot of pads because I use a generic controller from HKSUNWIN on ebay, there is an entire thread on those controllers, how to set the LVC by the resistor on the pcb. Another product you can modify for more power!
The good thing with the Leaf is that I believe its plug and play. Also note that its better to buy the motor itself, and buy good quality rims and spokes and get a shop to lace it up for you.
Rim would be 36G and double wall like DM24 Alex Rim or Sunringle MTX.
Double butted spokes 13-14 Gauge.
A decent tire like the Marathon Plus. Torque Arms.
Digital Multimeter.
A couple thumb controllers, I like Greentime on Aliexpress you can use the on either side L or R.
Soldering Iron, Solder
Spare connectors, I like the XT90's from HobbyKing but the Anderson Powerpole PP45's are good.
Zip Ties
Duct Tape
I do not know if you can specify a Turn Count with Leaf or not.
Leafbike.com
I bought a 4T because at first I wanted a lot of speed, but I realize now I do not want a lot of speed, what I want is a ton of torque, which having a 3T or 4T is good because to get that torque you reduce the diameter of the wheel, which in turn nets you less speed.
You adjust that buy the controller and battery that you buy.
The more battery voltage the faster the motor will turn. But your controller needs to be able to handle that voltage, and be able to pump out a lot of current.
Lots of people lace their motors into a 24" or a 20" rim, you get less maneuverability over pot holes so you just have to know what you ride.
City commuter in traffic - a lot of stop and go, go fast and stop type riding.
City commuter on pathways - slow yet determined speed as to not upset pedestrians.
Casual pathways -
Casual residential roads
Lots of long steep hills or a few hills.
All factors to consider.
