3 things to consider when buying treadmill to be used for running:

Stability, because runners land at 4-5 times their body weight.

Durability embodies wear and tear the treadmill may experience because it is used at higher speeds resulting in landing with harder impact. Another thing to consider is the duration of use on a day to day basis.

Suspension for runners cannot be too soft or too stiff. As a result of being too soft, there could be unstable responsiveness from high impact strides. And if the belt is too stiff, the impact could be tough on the joints and feet. Somewhere between a soft and hard belt would be capable of delivering the best results.