Hmm... there's really not much to a bike to be concerned with, part of the charm if ya ask me. Other than the wheels, the Walmart cruisers are fine candidates for motorizing - there's little point in worrying about the quality of the shifters as you'll hardly be using them, just about everything else is easily and cheaply replaced should you not like it.
The weld quality on the frame is a good thing to check, but really - it's just the rims/wheelset that are the only real weak point for just a commuter.
I purchased a walmart bike for my first build ($200 Schwinn) simply 'cause I didn't know anything about these kits at the time and didn't wanna invest too much. Once I decided I liked it I went and got myself a much nicer bike, you could do much the same if you are concerned about costs (even the $100 Huffy Cranbrook) - then keep an eye out for a better bike as you ride & learn, replacing the whole bike or just the wheels at your discretion *shrug*
It's when ya start gettin' fancy with shocks and disk brakes that a bike will start to add up
As for the rear sprocket - just about any 36 spoke wheel will work, the exceptions are rare enough in this price range I don't think it'll be a concern. Even many three-speed hubs & coaster brakes will work... with a touch of finagling heh