Recently stepped up my interest in the E46 M3.
Have been looking at what's involved in the service I and II.
Other than valve clearance adjustments the servicing doesn't seem to be any different to a normal car (filters and oils/fluids, when due). The rest of the services are just checking everything.
Whilst I'd want the valve adjustments to be checked by someone who knows what they're doing, the rest is just bog standard stuff.
It seems the key thing is to find one that's a) had the clearances done b) compression tested (or do it myself) and c) ideally had clutch/water pump/possibly starter motor changed as they're getting to the age where these items are more likely to fail. Obviously the rear subframe needs to be checked too and I'm sure I read on here that the rear brake lines are a common problem.
Certainly food for thought - unless I'm mistaken in my research.
The valve clearance is done every Inspection 2 and isn't that much of a difficult task in itself, it's just time consuming hence why an Inspection 2 costs the most. Your best bet would be to find your nearest specialist garage as they will charge less for labour. My local is £35 an hour and as such an Inspection 2 costs less than £550.
On all my cars I've had an oil service every year regardless of mileage driven and the sump plug and washer replaced each time too.
Maintaining an E46 M3 can be very costs friendly, you just need to look at it wisely and most importantly, find an indy that takes some pride in what they do (this isn't easy I'll admit, I came across my local one by sheer luck browsing Google Maps one day!). Buy oil + filters from places like ECP. It's cheaper than what any indy can source and then all you're paying is labour really.
You're more likely to see through common faults such as failing sensors and brake/body components than actual engine issues providing the car has been routinely serviced.
The other little bit that makes these cars so fun to own is that because they've been around now for so long, the community is huge and you can literally plug in your laptop or smartphone and start hacking away customising in the things you want and removing those you don't.