Treadmills are like boats - the two happiest days are the day you get them and the day you get rid of them.
If you must get one - the most important thing to look for is to make sure the damn thing has wheels under it - that way it's easier to shove out of your way while you are cursing at it.
Seriously though - if you really want one - check Craigslist. You can get a new one for next to nothing, virtually unused with just a layer a dust on it -
Okay, really serious - I'd suggest an elliptical machine instead. Got rid of my treadmill and went with the elliptical. A ton less noise, a ton easier on the knees, shorter workouts - and - you may actually use it like I do mine.
Ron