If you're going to buy a long lived reasonably economical car, no one thinks about the 7-series BMW. Not of any vintage. All 1988- cars suffer all kinds of maladies. Only the 3.5L six equipped 735i's have decent drivelines, but most are auto-box equipped. ZF's BMW boxes are lousy. Porsche got smart and bought MB's auto-box for the 928.
The electrical nightmares that the 1988-1995 E34 5-series suffer are worse in the 7. The V-12 variants can be had for a song thanks to their complexity and terrible reliability.
Any of the older M cars can be a financial disaster too. The engines are essentially race engines, and are overly complex and parts are all M-specific and EXPENSIVE. Enough to make Porsche drivers gasp. MB "special" stuff is usually "generic" in it's way. Look at the W124 E500. Most of the car is run-of-the-mill W124, and the performance bits are from the MB parts bin, and don't cost the GDP of a decent sized nation. Go price out some M engine bits, I guarantee you'll faint.
The 1984-1991 BMW 3-series (E30) cars are well made, and can be great cars, but they're all very old now. Most are third, forth, of more owner cars, and are past their useful life. The early E36 (1992-1998/99) 3-series are good if they've been sorted out, but the best used BMW buy is a 1998 328i sedan or 1999 328is coupe.
We had a 1997 (E39) BMW 528i, and sold it to my neighbour. She loves it, and it's deadly reliable compared to the crappy Chrysler products they've had in the past, but not any better than the 1998 E300 that we sold out of disappointment. It depends on your expectation. One thing I have noticed is that the BMW interior materials are not as robust as in the past, and compared to my C-Class, the E39 interior is already looking a little peaked, despite much lower mileage on the BMW.
BMW has driven down the mass market road. They've got models out to wazoo, and are expanding their SUV product line. There is talk of 1-series in North America. And so on. They are also following Lexus' lead in cramming gadgets into cars, but can't get them to operate reliably. The Germans don't have a good track record of being all things to all consumers, and all the german makers are failing miserably. VW's are regarded among those in the know as the worst junk except for a few NA cars like Neon (an anagram of None) and so on. Audi has built a good rep, but that might fall once their ailing quality numbers sink in. And service? Audi's dealer network sucks as bad as VW's. Porsche has been on a roll, but Boxster sales are off big time, and there are plenty of unhappy customers with broken engines, and tons of little problems. Remember, mass market consumers moved up from Camrys. They're not willing to put up with crap.
Right now, I would be very hesitant to take a chance on the large outlay for a new Mercedes product. Check my sig, and you'll see that is a problem for Mercedes. The die-hards are walking away, and they're left with the "fickle" mass market that'll turn the ML in at the end of the lease and not bat an eye when picking up their new RX330.
1998 C230 "Black Betty"