Now that we know, of course you MUST build your own. And now that we know your budget, you have no choice really.
Get a half decent FS bike, if you want to off road much. In the walmart junk category, the Genesis or Mongoose with the rocker type rear suspension link is the thing.
http://www.walmart.com/ip/26-Mongoose-Ledge-2.1-Men-s-Mountain-Bike/26999421
Don't get me wrong, this bikes front shock is total junk, but because of that rocker arm, the rear will not be too bad. Look for a model with a 1 1/8 headset vs a 1" so that front shock could eventually be upgraded.
Even better, might be a used MTB with only front shocks, but a decent fork on it, especially if you won't really be riding off road the most. In forks, the cheapies have no adjustability. Better entry level forks will have preload and rebound adjustments on them, and longer travel.
There is an ES member named Veloman in Austin, who happens to be selling some used stuff. You might be able to get a bargain on a motor from him, then add a cheap controller and throttle. Look in the for sale used section here. Or just go with the affordable 1000w kit from YES.com
If you spent $200 on a bike, and $400 on a kit, you'll still have $400 for a battery and charger. That's tight, but you are on the way to being an engineer. So you can handle it to use radio control lipo batteries, or scavenge batteries to build your own. Start with 48v.
http://www.hobbyking.com/hobbyking/store/uh_viewItem.asp?idProduct=18631
six of those would make you a nice 12s 44v pack of 10 ah. This came up hong kong dollars on my browser, but it should be less than $30 usa each.
Don't screw up with the lipo, if you do it could burn the dorm or apartment down.