Hello!
Every spring or so, Consumer Reports does an analysis of health equipment. I would recommend going to your local library to check out the following issues as they will have details on popular models and a "primer" on what to look for.
Feb. 1999 recommended treadmills:
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Health Rider Soft Strider EX
Life Fitness TR-4000
Feb 2000 recommended treadmills:
Health Rider S400
Reebok ACD4 (folds for storage)
Tunturi 5660
Mar. 2002 recommended treadmills:
Life Fitness T3 or T3i
Reebok ACD4 (folds for storage)
Precor M9.33 or M9.35
Star Trac TR901
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Once again, go to the library to check out the full details for each model. They also have a section on whether to buy used and how. I would recommend browsing "Play it Again Sports" (used sports shop) until you find a model that suits you.
However, I would also recommend that you begin by walking around the block before buying a treadmill. Treadmills have the highest usage rate among purchasers after a year (~55%) but that is 45% who don't use what is a very expensive machine. Also, remember that you need to add 4% to the grade to compensate that you are stationary (and not moving forward). 4% equals a flat road and less than 4% is the same a walking/running downhill.
Finally, I would check the weight rating on these if you plan to practice carrying a pack or ruck on the machine. Some have weight limits of 250 lbs or less.
HTH,
James