M10 and M30 engines share the same basic architecture (the M30 just has an extra couple of cylinders), so much of what follows comes straight from the 2002 world, but is generally applicable to your M30.
The M30 is about the most stout, long-lived engine BMW ever built. Even at 240k miles, there's no particular reason to open it up unless it is exhibiting specific problems.
The head is the weak point in the engine, inasmuch as it has a weak point. Most everything in the head can cause problems: the occasional broken rocker (more likely if you run at high RPM a lot), worn adjusters that allow clearances to creep easily out of whack, leaky valve seals, valve springs that go soft with age (again, lots of high-revving will make this more of an issue), oil spray bar bolts that work loose and starve the cam lobes of oil, and a general tendency to warp quickly if overheated. Note that the head bolts are reusable and should be checked and retorqued as necessary before each valve adjustment.
The timing chain will stretch with time, and the spring in the tensioner will get weak. The water pump and thermostat will eventually fail (though less frequently than on newer models). The ignition components are usually good for 60k miles or so if you use the factory parts.
The electrical stuff - switches, relays, solenoids - can be flaky as they get old. You might preemptively replace all those on the engine just to remove the possibility of an inconvenient failure. The thermo-time switch, the cold start injector, and the main relay are the biggest culprits; I'd replace all those at the very least.
The bottom end is so strong that there's no reason to touch it. The only thing I'd even consider doing down there is the oil pump, and only if I was seeing marginal pressures.
In short, aside from a few wear items, I would inspect carefully but not do any more than was necessary. If you do go as far as installing a cam, you should then completely overhaul the head; it doesn't make sense to do all that disassembly and not refresh everything.
