The best treadmills in the industry are made by Precor. They aren't cheap ($2500 and up), but they last the longest and are the least likley to cause injury. We couldn't afford one when we bought ours, but that is what I would buy now if I could.
We have a Reabock Treadmill. Mandy runs on it every morning (I prefer to run outside). We are very happy with it and it has held up very well for 3 years now. It wasn't cheap (around $1300), but it was worth it. I good medium price treadmill.
My recommendation is not to try cut costs on this. By a new tradmill by a decent manufacturer from someone who will support it if it has a problem. It will cost a lot more than what you would spend on a department store or a garage sale, but you will get a serious piece of equipment that you can trust.