OK friends, I need recommendations. My Physical therapist has recommended we get a stationary bike for my knees. It needs to be something that I can use with NO resistance for a while, but I'd like to be able to add resistance down the road. It needs to have a back on it (for comfort). I want something sturdy and not rickety. I've been looking on KSL and there are a few I might be interested in. Just wondering what brands you've had experience with (both good and bad) and would I be making a mistake buying a used one?