I use to recommend those two:Viewsonic VX2450WM
and ASUS ML238H
Both of them are very nicely reviewed, you should get a fine deal out of those two. Now the feedback you asked for:
- There will be not much difference between 21.5" and 24" monitors, so get the bigger one.
- The response time is indeed unnoticeable.
- Contrast ratio is, in a nutshell, how black the screen displays the black color. The less the contrast ratio is the more dim your image will appear, and contrast ratio makes A LOT of difference.
- I don't like to buy used displays for they usually have dead pixels and stuff. But if you can find a good deal I think it's pretty worth it!