I have one too. I bought a pup but its sorted now.
Clunk could be anything - exhaust middle mount would be my first place to look.
Clutch
Is it totally smooth all the way down? If not it needs a new one
Engine
should sound like a sewing machine Any rattle its gonna Make sure you hear it from cold If it sounds like a diesel for more than a second - needs a new variator.
Cam belt
Max 4 years old or 40K miles any more of either and it needs a new belt. Parts alone will be around £500 if it fails. New belt costly because access is very tight and special tools are needed.
Aux belt
Cheap enough but a pig to tension (I have some tips)
Battery
Budget £100 for a top spec jobbie to keep steering happy in cold weather.
Brakes
Handbrake Calipers cost £mega so be sure they work well
Front calipers are about £100 each service recon as you cant easily get exchange.
Brake discs etc are otherwise reasonable to fix, but check for grooving
Other
Any resonant humming sounds from the back = wheel bearing Hubs are about £50 from S4P, £30 from ebay.
Radiator is about £100 if you are lucky but double that from S4P
Make sure the air con works. Dont fall for any "just needs gassin up" crap. The dessicator costs £100
If the front cats have gone just fit a pair of decat pipes.
But its great to use and surprises BMW drivers.