If there is nothing to say they have been sorted already first and foremost would be swirlflap removal. Follow that up with a new crankcase breather just in case that has been neglected. Other than that make sure it gets up to temperature and keep an eye on the glowplugs, if one of the thermostats (EGR or main) or the glowplugs fail it will stop a DPF regeneration because the system requires certain perameters to be met before it will activate a regeneration. Luckily the latter isn't such an issue for someone driving an old croker of an M57 like mine in my E46! Swirlflaps and crankcase breather were done within a few weeks of me owning the E46, I was taking no chances, in fact I'm pretty sure I ordered swirl flap blanks in advance of buying the car. Oil changes every 10-12k is my rule of thumb for servicing and not the 18k as suggested by the OBC. Much later down the line vacuum lines may need replacing.
Mine does need the main thermostat and glow plugs replacing which I'll get booked in very shortly, but even then it still starts up first turn of the key with 206k on it in current temperatures.
Generally, they are solid engines, I've seen an E46 330d on eBay with nearly 500k miles on the clock for example. They're not as refined as the later N57, but I just couldn't live with a higher mileage N57 day-to-day wondering if the timing chain is going to snap or if its going to spin a main bearing, which will sound over-dramatic to some, but that's me. Personally, I'd not flinch at buying another M57 engined car with around 100k on the clock.