ST,, I used to have a running treadmill, it died and I miss it. Running, meaning that the deck on it was longer than regular treadmills.

My advice and gawd I wish I had one at home,, is a Cybex Arc trainer w/ski arms. I don't do well on stair masters or a lot of eliptical machines <--they actually hurt my knees. For whatever reason the Arc trainers do not. I prefer the ski arms vs the other kind of arms. Once acclimated, you can do leg work and leave your arms by your side. Then dig in if you get in a rut. Price tag is approx $1,200 used. Takes up about the same space as a treadmill though because of the base.

Once you figure out the style and brand you lije, check with the stores that have used gymbequipt. They have high end and low end. Just make sure you know it's age and then compare prices. Then watch for sales.