I'm sort of a newbie too, but I can answer some of your questions. No need to take anything from your engine kit with you when you buy your bike. 26" refers to the diameter of the wheels. If you buy a men's 26" bike you will generally have room for your engine kit. Do some extensive research in the forums here and you will find a wealth of knowledge about which bikes work well. As for the number of spokes: your kit sprocket has 9 holes in it and it comes with what some folks call a "rag mount". In order to get your sprocket to fit you need a 36 spoke wheel (36 divides evenly by 9 that is why). The type of hub that is on your new bike is slightly important. If you get an internal shifting hub (like I did) the you will have to make the hole in your sprocket bigger to get it to fit. Single speed hubs or hubs with cassette type shifters usually are the right size to fit the sprocket without modification.
I bought my bike online and I'm very happy with it. I got a Micargi Rover (3 speed internal shifter) from www.bikebuyers.com
and only paid $250 with shipping.
I wish you luck in finding what you are looking for and luck in building it.