Are you looking? I found a 1990 560SEC here in Orlando, 83K miles, 12,900 bucks. Light silver car, parchment leather. Dealer was closed, but it looked nice from the outside. Look around, there are plenty of the 560's in that age range around, and you do not need to pay $20K for a nice one.
My 1989 560SEL needed new accumulators for the suspension in the rear, and a timing chain at 127K miles. At that age, consider the timing chain and guides a requirement. Other things are the same as your 500SEL, as far as all the other systems, (a/c brakes, etc.) Just go over it very carefully. And look out for trans leaks..they are big bucks to repair!
Good luck, and sorry to hear about the 500SEL getting keyed.
1989 560SEL (127k)
1971 Cutlass convt. (98k) (sold)
1978 Olds Toronado (68k) (sold)
1977 Camaro (sold)
1984 300ZX (146k)(sold)
(only a 2 car garage explains a lot....)