Sorry for the delay in responding, you are probably sorted out by now.
I would agree with Dave that the exhaust is unique to M5. There are numerous system type variations within the E28 model although many mechanical parts are of course common to the equivalent 6-series.
Whilst BMW parts can be expensive, they are normally long-lived and of course there's no hassle about them fitting properly. Your preferred approach may depend on how long you intend to keep the car.
If its' only the front or rear box that's failed, it might be worth fitting only that box - depends what the other one's like.
Parts support for this model seems good - whilst only fast-moving stuff is held in the UK (a relative term for an M5 - as there were only 187 GB cars), a dealer can get stock from Germany within five working days, sometimes less. You will need:
There are also numerous brackets, straps and fixings which you may or may not need depending on how badly they are corroded.
Note that the system ex-factory is fitted in one piece, to replace a part-system it is cut in the vicinity of the rear axle, hence the need for the welding sleeves.
Caution - the above numbers have been taken from an old parts CD so you should get the dealer to check the numbers are correct before ordering.
You will need to quote the VIN number when ordering parts - just the last seven digits should be sufficient. On that subject, I'm trying to get a picture of just how many of the 187 GB cars are still in circulation. Perhaps you'd be good enough to send me a PM with the number and the spec of the car?
Its not a near concours alpine white one is it with cloth trim and about 77000 miles on the clock? I lost out on a gorgeous alpine white car when it was on ebay about 6 months ago. The guy selling it was in Chiswell Green, St albans. It was a cracker
the car i have bought is nearly concours it has covered only 3000 miles in the last 8 years it hasn,t been used for 6 years ,it has 104k with black leather and is absolutely perfect ,no rust , its number 45 ,the engine was removed to do the timing chain and the engine bay was cleaned at bmw dealer ,i am just cleaning under neath the wheel arches they are white with no rust anywhere i am thinking of showing the car its that good.