I bought a cheap Weslo a number of years ago. We used that one until it wore out (DH is great at fixing things and he fixed it a couple of times, but there was only so much he could do.) So, we then bought a used NordicTrack from my sister-in-law, who had hardly ever used it. The main problem with the NordicTrack is, after I had used it for 6 months or so, it started stalling whenever it's set at 0 incline. So now, I can only use it if I set the incline to 3 or more. (The cheap Weslo I had couldn't even be set to be flat, you had to use it at an incline.)
In general, it puts more stress on the treadmill if you use it flat, rather than at an incline. When you walk at an incline, your own body weight helps to move the belt. So, if you don't want to walk at an incline, you will need a better quality treadmill. Also, the faster you want to go, the longer a belt you will need. If you plan to walk rather than run, you can get by with a smaller, cheaper treadmill.
I think buying a used one is an excellent idea. There are lots of people who have treadmills, but never use them, and may want to sell one. I would suggest asking to try it out first, to make sure it works.
I second the advice to check out treadmilldoctor.com I found them very helpful.