Okay that sounds like a common bmw owner ship.I've encountered all of the multiple problems with the n54, multiple times. The biggest problem for me was the rear main seal which went at 190k, and the oil pan gasket which went early on at 120k. I've replaced the injectors one by one. Haven't touched the water pump, but the head flange broke on me last month. The suspension is needier.
The block and tranny are bullit proof but everything around it fails.
I also had,
valve cover gasket leakage (due to the alluminium bolts breaking)
oil filter housing gasket to block leakage
radiator leaked between the alluminum and plastic fused part
also the coolant head flange leaked
changed the entire PCV system because due to the heat/age it becomes brittle and breaks.
Changed the large DISA valve because it was worn (n54 doesnt have that)
I also changed all the coolant hoses/pipes but more for prevention because some of them where affected by oil deterioation.
Rear main seal and oil sump leak only a verry little amount and have been doing as long as i had the car so probably wont touch those until it gets worse.
My waterpump is also still original, however after 15 years i do think about changing it before it fails on me in the worst possible location.
I can do everything my self so it doesnt cost a lot but if you need to bring it a garage everytime to fix these things it will cost a fortune to drive it.
Also with the N52 these are all very cheap parts, i think the most expensive was the DISA valve and radiator. At maximum 200 euros a piece.
However with the N54 the 6 injectors cost 1500, the fuel pump costs 1000 the 2 turbos cost 3000
However looking on all the forums and all the reading that i did i dont think milage effects these problems a lot, its more age that does it.
Its not like that if you pay top dollars for a 60k milage car that you will be free of problems.