Stay away from something with gobs of bottom end torque. You're a newbie, your throttle control is going to suck, and there are going to be times when you accidentally snap that throttle open, say going over some bumps. On an older inline-4 or smaller v-twin, you won't really notice it. On a modern sportbike or one of the new "cruiser with tractor motor" type bikes, this will quickly get you into trouble.
My advice? Go out and get an 80s era 750cc inline-4. Honda, Yamaha, Kawasaki, and Suzuki all made good bikes like that. Learn on it, have fun, get comfortable, even ding it up a little (it happens), then resell it for the same or more than you paid for it, and get something nice. They're reliable, easy to wrench on, and they DO have good power but it's up high in the powerband---this translates to fewer "oops, too much throttle" incidents.
Before I got into ROTC, I worked as a motorcycle technician at a Honda dealership. I'd see so many brand new first bikes go out all pristine, just to end up on my lift a week or two later. For 95% of newbies it isn't a real crash that gets you, it's something dumb like dropping it while stalling, or coming to a stop, or stopping on sloping pavement---the infamous 0mph crash. No big deal on that old '83 Honda Magna, but it really hurts when it's your brand new Suzuki M109R...