M3 CSL - luverly. Hard core E46 - what's not to like. I miss the sound of the S54; the 981 BGTS is nice but doesn't have the symphony of induction, mechanical and exhaust noises of one of BMW's best straight 6s.
Very different car obviously but a friend owns the 99 E46 328i that I had from new back in the day - after 20 years and 195K miles it's still a joy to drive. Just watch the usual E46 rust traps - it's hugely disappointing that BMW dropped a few clangers on the design/build of the E46 that leaves them with rust traps in the wheel arches and under the sill covers.
I had the afore-mentioned Z4M coupe and an E61 M5; my experience with main dealer servicing on older M cars was patchy. You'll need to find one that has M certified techs on cars of that era; many will have left for independent garages or even set up on their own.
Iridium is reasonably close to me - let me know if you need a lift to the station!
There's also someone in Haslemere (names escapes me) that looked after a friend's ex-DTM E30 M3. And https://walkersautotech.co.uk is another option.