The harmonic balancers go and make a noise like a small exhaust manifold leak under moderate acceleration. The only real fix is the ATI, which isn't cheap, but is top notch.
The strut towers mushroom easily, and you'll know when it happens--I hit an expansion joint on an offramp and my steering was immediately off-center. The IE fixed camber plates are great, and you can easily fix the mushrooming with a piece of 2x4 and a hammer. The IE plates are cut, not stamped, and cover about twice as much of the surface of the bottom of the strut tower as the stock mounts. I would consider this PM on any R53.
Change the serp belt often. PM goes a long way.
They like Castrol Synthetic.
They will eat HID ignitors, which are integrated into the lamp housing and not serviceable. --Buy used ones, as they're 1k apiece new.
They eat electric PS pumps.
If you get a notchy feel in the steering on-center, it is the u-joints on the funky triangular aluminum steering shaft. Mine was replaced and lasted less than 20k miles before it did it again. I was able to vastly improve it by overtorquing the set-bolt on the joint behind the steering wheel.
Window regulators are 50k replacement items. I think it's due to them getting worked every time the door is opened, closed.
Windshields....find a way to get a damage coverage. I got mine bullseyed the day after it was replaced. I actually just got a class action settlement notice in the mail for R53 owners that had a windshield done at the dealer with less than 50k miles.
I had a white vinyl wrap material on the sunroof that had little holes in it. Reflected the sun and you could still see through it and get light in the cabin. Helped the meager AC.
These cars are wicked fun, if you can adopt the same maintenance philosophy of a BMW or Audi.