I think the most important thing is the motor. If that's no good then the rest of the features are meaningless.
Ours has springs/shock absorbers under the track. If you're planing on running not just walking they'll make a big difference to your comfort.
I wouldn't buy one from ebay - the reason is that if it needs repairs under the warranty you might have to pay a fortune in shipping.
Most of the companies bring out a new model each year so if you can get last year's model on special you'll get a good saving without having to compromise on features.