Stay away from one with "Bells and Whistles"....there great if you're a gadget freak, but for the average person who just wants to get on a treadmill and run, they're next to useless. Basically, try to get one that has options that you'll actually use, and use often, like a heart monitor, manual controls for speed, incline adjust if you want it (which really works your legs). In all actuality, the manual controls let you adjust things on the fly, like speed, incline and cool down, so you don't need to spend the extra money for a treadmill that has all those things built into a program when you can do it yourself. If you have questions about your workout routine, look it up online, you can find anything out, like your target heart rate, fat burning heart rate, cardio heart rate, interval running/jogging, how long to warm up/cool down etc. etc. Just my opinion
You might also want to consider an elliptical trainer, as it'll be easier on your knees and back--there's no impact when using it unlike a treadmill. Just get it at a place where you can return it if it bothers your knees or back. My wife loves the elliptical trainer and goes forever on it. Me, on the other hand, it bothers my back so I don't use it.
Just remember to ease into a workout routine ("consult a doctor before beginning any workout regiment")
Oh, and get a MP3 player or crank the stereo.....it makes the time go by faster!
War produces veterans, wounded both physically and mentally. They have sacrificed for us.....and it is now our job to help these veterans, as they have already helped us in ways we will never know, in ways that we cannot fathom, and in ways that we take granted every day.