I payed around $80 for my 201 in a small table. I have never regretted it. It needed cleaning and a few replacement parts, and has turned out to be my favorite. For my first two machines I didn't mind paying for them at all, but I hesistate more now, unless it's something I have had my eyes on for some time. It's well withing what's normal for an intact working machine in a cabinet, even with a a few parts missing and cabinet in need of attetion. You could always let someone looking for a machine in a cabinet have it, and wait for a 500 in a case in stead.
Your 66 should be relatively easy to sort out, but I know tension can be a major fuzz at times. The early type tension with a simple nut is perfectly fine. Clean the top tension unit and make sure the spring on the bobbin case is clean and that nothing is stuck behind there too. Adjust the bobbin tension with a drop test (bag of rice or beans) and fine tune top tension from there. If the adjustment screw on the bobbin ccase release lever has been messed with it might need to be sorted out.