As the owner of an '85 300td I can hopefully help a little bit. First I think the asking price is a bit high so if they aren't willing to dicker a bit you may want to keep looking a bit.

As to your questions, yes they will roll more than a sports car but shouldn't roll a whole lot either. Figure that a used Benz probably rolls somewhat in the vacinity of a newer Taurus but not more. You might want to give the front end a bounce or two because you may only need new shocks, a fairly cheap & easy fix if you do it yourself. Properly set up these handle pretty well.

The shaking at idle is likely bad engine mounts, valves needing adjustment, and/or a variety of more complicated fixes that can usually be done by a semi skilled home mechanic at fairly minor cost.

The rust should be carefully checked out. This will be very expensive to fix if it has started to get away from you so look at it carefully.

As to the paint I believe MBZ used a clearcoat but I may be wrong, I'm sure others here will know for sure.

Hopefully this will get you started. Others here should be able to add to this. Good luck.

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LRG
1987 300D Turbo 175K
2006 Toyota Prius, efficent but no soul
1985 300 TDT(130K miles of trouble free motoring)now sold