Some problems that can arise in OM651s after 100,000 miles or more include: gasket leaks or warped rocker covers; oil cooler leaks caused by failed seals; diesel filter housing leaks caused by brittle plastic pipes; inlet manifolds that start giving trouble; and last but not least, engine timing chain stretch, which is indicated by chain rattle-on-cold-start. Also, EGR coolers can become clogged.
You could choose to get everything done at once so it's perfect, or you could fix or replace things as they break. For example, when my inlet manifold leaked oil, I had to remove the diesel filter housing and replace it at the same time. In my opinion, the two most important things are to change the oil at the intervals specified by MB (I change mine every 6,000 miles) and to inspect everything on a regular basis. The reasoning behind this recommendation differs from MB's 40K mile recommendation; according to another piece of advice I read recently, diesel filters should be changed every 20,000 miles to avoid injector problems in engines with higher mileage.