My experience of BMW has been very positive. They are generally extremely well built and a pleasure to drive whether on the limit or at tootling speeds. The 4 or 4.4 litre V-8 motor in particular is a delight. It spins fast and sounds great.
That said, the 7-er cars you're looking at are presumably old shape (?). If so, those cars are not as well built as an MB. If you are looking at present model 7-er cars, the build quality far outstrips current MBs. They really are bullet proof.
The comments about the dodgy cylinder bores are true, but at the mileages you're looking at, those faults will most likely have been remedied. BMW supplied new engines on a warranty or goodwill basis. In Europe at least, they were very keen to be seen to be sorting this problem no questions asked.
As for the badge-engineering that another poster warns of, you just can't badge engineer a 735i into a 740i: the V-8 car (740i) has, surprise surprise, a V-8 under the bonnet. The 6 cylinder (735i, 730i) has a very obviously in-line six. Also, the V-8 cars have the broader kidney grille of the 750i.
Personally, only the slightly iffy reputation and image of BMW and their drivers puts me off. I genuinely think their cars are much better built these days.
1999 SL 500
1993 320 TE
1986 500 SEC