You can get them very cheap if you have a way to get them home. When you find one you are interested in, google for a copy of the manual. There should be a way to check the total miles on the thing by pressing certain keys. Other than that, make sure it runs (duh) and the display works, the belt isn't frayed, the incline works, and it doesn't make any weird noises.
As far as features, look for one with a high weight limit (will be generally more durable) and a good motor (mine is 3hp I think). I don't use any of the programs on mine but it does have a heart rate monitor which is nice. I also love that it has one touch buttons for speed and incline (1-12), instead of having to press the little arrow up or down forever
If you have a used sporting goods store, they usually have several. This might be a good middle ground.
I loooove my treadmill (got it on super sale new at Sears) and am so glad I got it. You can go try them out at a store to get an idea of what features you like and don't like, and what belt size you prefer, etc.