There are lots of bikes that will work. A lot of folks get nearly new bicycles at garage sales and thrift stores for less than 20 bucks. So if cash is tight you might look at something like that. As far as what works best depends on the kind of bicycle motor you are going to use. Here's good place to start looking
Classic Instructions for Building and Repairing Motorized Bicycle Engine Kits - Bicycle Engine Kit Classic Posts at Motorized Bicycle Forum Read some of threads in that section. Be sure to read Bikeguyjoe's Motorbicycle preparation and care. Lots of good info there.........................