For that kind of money, if you are looking for "older" look for any Pentium 4 based computers. From time to time there are people here who are trying to sell their old rigs to upgrade to something better and they would love to hear from you if you stick around and keep checking back.

Also, you might be able to pick up an older Intel Duo Core "E" type processor based computer. Look for an E7500 cpu. People might want a little more, like $200 for this one, but the technology is better and the cpu is superior to an older Pentium 4 monocore. So if you can break for another $50 I think it would be well worth the investment.

For an older computer, be looking for these kinds of specs:

At least Windows Vista (avoid XP or older, beause Microsoft is not supporting those os's any longer)
If possible, Windows 7, if you can get a computer that cheap with W7
Hard disk - At least 320 Gigs and preferably 500 gig
Video Card, 9800 GTX or ATI HD 4850 card
DVD drive
At least (minimum) 2 Gigs of RAM - try for four gigs if you can find it.

I think if you scout around a bit you should be able to stay in the $150 - $200 range on those parts.