the bike should be preferably v frame,with gears,because the sprocket wont fit well on a coaster brake bike(the kind u pedal backwords to stop)
You should probobly buy a chinese kit,those are the easiest and cheapest.
All the Chinese kits are basically the same.here are some I would buy.
(look though the options buy a gas not electric)
the ebay sellers have supposedly bad costumer service but i bought a part from one and it came ok.
Or you might wanna consider a Goped or a moped becuase those are better quality and they are put together already.
just as a warning though a motorized bike kit isnt fast,so in my opinion the safest place to ride it is on the sidewalk,