d8veh, did you say the 8 speed cassette will fit onto the Q100H?
And: you mentioned BMS sells the Q100H already in a wheel? (I can't seem to find that on their site). If not, will I be able to install it in the original rims?
(Kinetix Comp 406x17)
Are there any other motors I should be looking at?
Are there any step-by-step walktroughs or checklists for a similar conversion to be read online?
8 speed should be OK. You'll have to add a washer or two on the axle because of the stack height. You might be able to do it by fitting an inboard anti-rotaion washer depending on what type they provide. Then, you'll need to dish the wheel a bit to get the rim central. AFAICS from photos, it has 32 spokes in the back wheel, so no, you can't use your existing rim.
Here's the Q100H in a rear wheel. You can only get 260 rpm, which is 15 mph in a 20" wheel. If you want the 328 rpm one, it has to be the Q100, which will give 19 mph.