Mark, if the idea of moving is a real possibility, then I suggest you forget the hidden yard and live with the cassette idea. Most of this layout can be built in sections to make moving easier, but chances are slim that they'll fit the new space anyway. Plus, if you do have to move for health reasons, what are the chances that you'll even be able to rebuild the layout?
As for the knee replacements, my wife had hers done before she was eligible for Medicare, but I don't know what your insurance covers. We had TriCare with the Air Force and they covered all buy some small co-pays. Most doctors will try to put off knee replacements until you're well into your 60's because chances are they'll have to be replaced again. Either way it's something you need to discuss with your doctor. He/she should be able to tell you how much it will cost with your current insurance and advise you on what kind of supplement you'll need if you put it off until you're eligible for Medicare, I don't know if Medicare covers the full cost either.
