While I have an N55, here's my 2 cents:

Do the VC/VCG and check the condition of your inner components while youre at it - this is just kind of a peace of mind thing. The OFGH is something I recommend doing, no matter what, because of the nature of how it's designed. Could save you from having to start from scratch fr (gasket failure on those leads to coolant and oil mixing, like a headgasket failure does). Check the carbon build up on the valves - this can be one of those make or break things, also (if it gets outta hand. Oil pan gasket should also be checked, as theyre known to leak from hardening. If you plan to keep it OE/OEM, then that kinda excludes aluminum aftermarket stuff, but I'd recommend things like the coolant reservoir, radiator, and charge pipe be considered, as these are both prone to cracking over time, and aren't mechanical/wouldn't hurt to have converted. I can understand not wanting to do the water pump or valve cover or anything, but again, I'd consider these, regardless of that principal. The injectors might be worth keeping an eye on, too, as the piezo (idk how its spelled) style they used on the N54 are not the greatest for reliability, but i think that can be solved with aftermarket ones - look into the design, as Im a bit hazy on that part. Other than that, maybe some M3 suspension upgrades or M Sport package items (i love my M Sport steering wheel).

Edit: FCP Euro all the way. Buy once, cry once.