Hi and welcome aboard (from another older member).
I saw you have 2 posts running, but I thought mine might follow on better from what Zedebee said, so here goes!
The M54 engine in the pre-facelifts came in 3 sizes, 2.21 with 170bhp, 2.5i with 192bhp and 3.0i with 231bhp.
The N52 in facelifts (2006 on) came as a 2.5i with 177bhp, 2.5Si with 218bhp and 3.0Si with 265bhp.
I've only had one M54 in a 3 Series but am on my 4th N52, 2 in Z4s and 2 in 3 Series. Personally I prefer the N52 in a sports car as it revs higher - to 7,000rpm rather than 6,500. Either way if insurance isn't an issue bigger is always better as the running costs are the same as for the smaller engines!
The electric water pump on the N52 will die at some point and when it does the engine will overheat within a mile. Whether you replace as a preventative measure depends on how inconvenient such a failure would be - new ones cost £250+. The pump on my 1st Z4 died at 60K miles but my 2nd was on the original when I sold it on 91K. My first 3 Series needed a new one at 135K, but for all I know that might have been it's second pump - they are a lottery, but that's why I have recovery insurance.
M-Sport seats are highly desirable (used ones sell for up to £1K a pair) and were a rarely specified option on pre-facelifts. But facelifts came in SE or Sport trim and M-Sport seats were standard on Sport models. All 3 litre Z4s had electric seat adjustment with driver's memory.
But standard spec was a bit mean so there are some desirable options, with heated seats being top of the list by all accounts for E85s to prolong the roof-down season.
Other options to look for may be Bi-Xenon headlights, rear parking sensors, wind deflector, rear parking sensors, Multi-Function Steering Wheel and cruise control although that last one is a fairly easy retro-fit.
One other thing to look for is corroded rear brakes pipes. They are in a plastic cover under the floor, but that ends a bit in front of the passenger side rear wheel and then end pf the cover traps road debris which absorbs water and can lead to corrosion.
Lastly the N52 engine can have "ticking" from the top end when cold, especially if the car has sat for a while but it doesn't seem to cause any problems. Getting the car properly warmed up and a spirited drive makes it go away.
It might be worth having a look at any classifieds on here as forum members do tend to spoil their cars. Anyway good luck finding the right one.
