My 88 560SL has 124,000 miles on it, and while it might have a little oil leak-down past the valve guide seals, it doesn't foul the plugs. You might try running a heavier weight of oil (20w50) for a while to see if that helps. Eventualy the valve guide seal will need to be replaced, but this might buy you some time.
Timing chain and rails: definitely change before 100,000 miles. I just had mine done for $500.
$800 for motor mounts alone seems high, but if he is also changing subframe mounts for that amount it doesnt seem to bad. Rubber mounts around the engine compartment don't last more than 10 years so they should all be replaced. I had all this done on my car early this year at an MB Dealer and it was right at $800. Hope this helps..