I paid $103 for mine on ebay. Gonna buy one for $119 next (free shipping on ebay). Spend more if you want but mine is great. Theres a ton of companies from what I see.
Starting... You lock the clutch lever so it peddles easily. When you get moving you give it gas and release the clutch. This spins the motor and it starts up and off you go.
You will prefer a bike with wider tires for a smoother ride. You will be hitting bumps and irregularities at 4 times the speed and thus, 4 times as many bumps. Ride counts big time. A bumpy ride on a regular bike becomes a brain rattler on a motorized bike.
yes, you want both brakes. And you want them working well.
Stop worrying about it. Its really simple when you get started. Just order a kit. 6 hours after you get it you will be riding around the yard saying... "woopy!!! That was easy".
Go to YouTube - Broadcast Yourself.
and search for "motorized bicycle". There some great videos that explain a lot and will get you really excited.
Now, go get that motor.
2 real popular ones...
49CC motorized BICYCLE ENGINE KITslant steel motor bike - eBay (item 150361746479 end time Aug-25-09 10:55:25 PDT)
LATEST JET SILVER 80CC BICYCLE MOTOR ENGINE KIT BIKE ! - eBay (item 380138202195 end time Aug-09-09 10:12:47 PDT)