Depends how worried you are about the bikes looks after you dump it. If you get something with body panels, and screw them up, it will take money to fix. On the other hand, panels can keep a lot of other things from getting messed up. Also, it's a lot cheaper to replace a tubular bar than clipons.
If you get something a little newer, try to do what you can to protect it. Get case guards or a crash cage (assuming you're not going to try to be Valentino Rossi and use up the ground clearance) for it, like the one's that the stunters use, to protect as much of the body as you can, and some frame and swingarm sliders.
Of course if you buy a really cheap bike than there wouldn't be much point in spending much money on it to protect it.
Fact is that dirt riders make better street riders anyway. So, if you just take her easy, maybe you can avoid the whole crash thing, and leave that for the dirt trail. Good luck.