Hello,
I can only agree with the other posts that a W126 300SE is a bomb-proof car, aside from normal wear items it should be trouble free. My friend in Raleigh has a 1990 300SE and I drove it down to Charleston and back in a day, cruising at or near 100mph all the way
It needed the lip seal at the front timing cover, exhaust manifold gaskets(DIY) and the valve stem seals(shop)replaced and routine maintainence
There may be a *clonk* in the driveline when shifting from P to D or R, but I was told by indies that it is the rubber mounts for the differential as the facelift W126s have a completely rubber bushed mounting for the diff, earlier cars having a solid mount. You could perhaps use this as a bargaining point :p
Have a good week.