I agree with Joseph. It's probably too late to do anything along the
lines you are thinking that will significantly impact the car's life or
be cost effective. One additional thing to make sure of, and that is
that no water accumulates inside the car from any leaks. I'm in the
process of fixing a couple of leaks in my 80 300SD. Water can get into
the car from A/C drain leaks, windshield leaks, etc. A little usually
isn't going to kill it. However, if it gets down into the floorboard
area, it can just sit there and you may not even be aware of it. The
carpet is so thick in these cars, it can hide it. You should also
check inside the trunk after a good rain occasionally too.
I recently pulled some parts off a salvage 300SD and noticed both the
driver and passenger floorboard areas were totally rusted through. I
was left wondering if that happened from the outside in, or the inside
out. My guess is it happened from the inside and may have been what
killed the car.