We have a lot of 250 5-cylinder diesels here. These engines are to be found in the W201, W124 and W202 models. The non-turbo version of these 5-cylinders are fairly good and last quiet long (although not half as long as the 4-cylinder 200 Diesels). We have W124 250D's with more as 800.000km (third manual gear box and second engine since 50.000km).
The head gaskets of these 5-cylinder engines tend to blow once every 300.000km.
The turbo version of the 250 Diesel needs more maintenance as the non-turbo versions. I have seen a lot of these turbo engines with blown head gaskets or worse, with cracked cylinder heads (somewhere between 200-400.000km).
The 124 chassis is a good chassis; no major problems to be expected with that.
Replacing the damaged 300SD?
with a minor 600+k