I have a 635 which has been off the road for a year while I welded up the floor - detatched itself from the sills both sides plus the obligatory huge hole in drivers footwell. It was only a few days work but I have too many things to do! The E24 can rust better than a bit of bare steel left in the sea, so even nice looking ones have holes underneath.
Exhausts cost £1k for BMW parts, £5-600 for aftermarket. Lots of parts interchangeable with e28 so not too bad. I got new front springs for about 50 quid as I recall.
I am getting mine back together, and intend to get it to serviceable status rather than pristine, might sell it or just keep it until something else major falls apart. They are great cars to waft about in, but unless you blow tons on an M it isn't a performance car ride quality/handling.
either buy a very cheap one as a resto project or spend the most you can for the best car. There's a nice looking one on bigcoupe at the moment...
http://bigcoupe.com/phpBB2/viewtopic.php?t=9388
Fuel cost isn't too bad, mine always seemed to sip fuel compared to the Audi S4 I had at the ttime!