A big complaint of treadmills that fold is their stability, many older models did sacrifice stability, and cheaper models will be less stable because of how they are built. Stability and durability is very important when it comes to treadmills because a shaky treadmill will jar you and lessen the effectiveness of your workout. When you are looking at different folding models you should test as many as you can out to see how stable they are and how well they absorb impact.
When you are looking at folding models you should also take the weight and how easy it is to move into consideration. Folding treadmills range in price from $500 to $3000, so there are some options for you if you are on a budget. But less expensive treadmills might not have the highest quality parts and construction, so you have to take that into consideration when looking at folding models. You can find folding models at several stores that sell exercise equipment and on numerous websites.
If you will be moving the folding treadmill a lot, you must consider its weight capacity. As all or almost all folding treadmills are designed to move, see to it that you won't have a hard time folding or carrying it.
Almost all folding treadmills are designed to move, the difference is the effort required. If you plan to constantly move the treadmill, check out how easy it is. If it is a 200 lbs. treadmill that requires tilting it on two wheels in order to move it, you are going to need some strength training in addition to a cardio workout. There are some models that when folded, have four recessed wheels that pop out, making it simple to move.
You can assume that all manufacturers are going to exaggerate the weight capacity of their treadmill. It is a standard practice. Whatever the capacity states, minus at least 50 lbs. You want a machine that can withstand the impact of your weight. Otherwise you'll find the motor, belt, rollers and bearings will burn-out prematurely.