parking brakes on 1999 Mercedes-Benz S320

was told my parking brakes cables were frozen and would take 5 hours for repair at over $900.00?

If you don't believe that's right, find another Benz specialist and get another estimate. I can't say for sure but it's believable to me, MBZ's are expensive cars and expensive to repair.
If you want to find another specialist, here's a directory link for you: