I'm also a big elliptical fan, although I don't have one - I workout in the gym, on a Precor machine, which is great. Like you, I find it especially easy on my knees, which are quite sensitive and tend to be inflammatory. I also run alot, but take it easier when my knees hurt too much, in which case the elliptical machine is a good replacement.
If you don't care about the money, look for a Precor machine, which I think is the best. It has multiple levels, intensities, slopes and programs, plus a smooth pedalling motion which is not trivial to accomplish. Maybe you can find a second-hand machine...