Check for oil leaks from the cylinder head gasket at the front and rear of the engine. Annoying, but probably something you can live with for a while. Keep an eye on oil consumption. Valve seals on these engines start to leak around 100K-150K miles. Not difficult to replace. MB supposedly used better valve guides in later M103s so that shouldn't be a problem for you.
It's common for serpentine belts to fail around 100K miles. The belt tensioner indicator will be spot on but the belt will be loose. Doing this job yourself will make you wonder if MB engineers have a conscience. Not difficult, but certainly needlessly complicated.
If the shift lever is loose or squeaks, it's time for new shift rod bushings. Doing this job yourself requires creativity more than technical expertise and special tools.
Search the archives on 300SE for other things to look out for.