Also make sure that you can adjust the adjustable belts yourself. Quality treadmills have pretreated decks requiring very little maintenance other than wiping off dust now and then. Look for one made from layers of different specialty material with a permanent wax compound rather than compressed fiberboard. A treadmill with good resilience absorbs the force from the impact of your feet, which relieves stress on your knees and ankles. Roller size is better if it's about 2 or more inches in diameter.
Solid rollers are much better than hollow ones, while steel rollers are better than aluminum ones.
A quality incline should be quiet and not cause the treadmill to wobble at high elevations. An electric incline is far superior to a manual crank, although it does increase the price. Avoid manual cranks that are not located right on the monitor console where you may adjust it while exercising. You can save money by purchasing a treadmill that offers only basic programming. More than anything, the control panel should be simple to use and easy to read.