Well maintained, 400,000 mile M30s are common. Ours has 300k so far and the head has never been off of it. It gets oil every 3-5k and the banjo bolts on the valve sprayer bar are checked every 15k, when the valves are adjusted. That said, there are seals leaking all over the place (see below), and I'm probably just going to swap in another motor. There are lots of good M30s out there for cheap.
Add main rear seal (trans/engine) to that list, though massive might be too strong a word. A complete set of all those gaskets is ~$350 and requires engine removal/and or lowering subframe. You can live with the leaks for a while, but the oil will wreak havoc on the E28's notoriously sensitive front suspension bushings, so if the subject car has minor leaking and front end shimmy around 50-60mph, the engine will have to be dealt with first -- otherwise, you're wasting money, knuckle skin and blood on bushings.
Any well-maintained E28 535 is a bullet proof tank. We've had ours about 5 years now and it's been a very reliable ride. If you're willing to turn wrenches yourself, it can pretty economical, too. Aside from the upcoming motor replacement, it's cost me less than $1,000 in parts to go 100,000 miles (radiator, water pump, starter and a bunch of bushings). A 100-150k replacement motor is less than $500.
Lots more 535i/is info here:
http://www.m535i.org/e28info2.html