Given the unique situation, I would find a good repair company that
works on Sub Zero. I'd have them come out, figure out what's wrong
with it, how much it would cost to fix it, and what the typical life
left in it after that might be. If it needs a new compressor, ask if
the new one will be more energy efficient, etc. Also ask about the
interchange of door panels to other units. If these panels won't fit
current models, it's possible they will fit other older Sub Zeros and
if you can wait awhile, you might be able to locate a used one that you
could sway out.
Normally, I would say junk it. But in this case, it's certainly worth
the cost of a service call to get some accurate info.