Good choice, but where do I start? :lol:
E85s you have the choice of 2.0i, 2.2i, 2.5i and 3.0i engines plus the Alpina, then facelifts with 2.0i, 2.5i, 2.5Si and 3.0Si engines plus the M models. So deciding which you want is a good starting point!
Personally I'd stick to the straight 6 engines because it's what makes the Z4 unique. Which rules out either of the 2.0i versions.
The early 6s used the M54 engine in different sizes, so running costs are the same. The facelift 6s used the later N52 engine that came in 2 sizes and 2 levels of tune in the case of the 2.5 - and the 2.5i can't be uprated to 2.5Si power levels AFAIK. Cooling systems can give problems with bowed radiators and split expansion tanks. They can leak oil from the cam-cover gasket and oil filter hosuing gasket. The CCV system can also get "gunked up". The N52 also has an electric water pump which doesn't leak or get noisy, it just stops. Then the engine gets too hot in less than a mile! Which isn't a big issue if you have recovery cover, but new pumps cost £250+. I had one die at 60K miles on my first E86 but my next one seemed to be on the original at 90k, so you should only ever need a new one once. But both engines are capable of well over 100K if looked after properly. The N52s produce a bit more power and rev a bit higher (7,000rpm) than the M54s, but which you chose is likely to be determined by budget.
They are pretty solid cars, but at this point in their life can suffer from rusty wheel-arches and tailgates on the Coupes. They are all prone to corrosion on the rear brake pipes just in front of the passenger side rear wheel. Roadsters also suffer from roof motor issues because where BMW located them they often end up sitting in a puddle of water from the roof drains. Luckily there are a few people on here who can relocate them into the boot out of harm's way for sensible money (less than £200 I think) and often revive "dead" ones.
Early E85s only came with one trim level, but facelifts (2006 onwards) came with SE or Sport trim. The Sport trim gets you a few M-Sport bits as standard, the most desirable of which are the M-Sport seats which can change hands for up to 4 figures a pair! Although I have seen SE models with them as an option.
The standard spec was a bit basic considering what BMW charged for them, although all 3 litre models had electric seats with driver's memory I think. A handy feature if anyone else will be driving the car. The most popular option on a Roadster is heated seats to extend the part of the year where roof-down motoring is comfortable. Other popluar options are Bi-Xenon headlights (the halogen ones are pretty poor), cruise control (although that can be retro-fitted pretty easily), rear parking sensors, Multi-Function Steering Wheel, upgraded stereo and cup-holders (but you can buy them). Sat-Nav was an option but it's so outdated your phone will do a much better job!
Mileage isn't an issue with either of the 6s, it's just ancillaries that don't last as long so with that in mind I wouldn't worry too much about mileage. Condition and history is much more important.
I hope I haven't forgotten anything, but if I have I'm sure someone will notice! I know I haven't said anything about the 4 cylinder models, but as I've never had a 4 cylinder petrol BMW I know nothing about them!
So once you know what engine you want think about colour preferences and any "must have" features then get searching. Good luck.
