Stay away from the old M70 V-12 as it had issues that affected its efficiency that you will not want to deal with in a car that weighs over 4,400 lbs.
Mine has the M73 which is the 5.4 litre, and has the most trouble free advanced control systems and design.
Although compression increased to 10.0:1, the valve train actually has less drag and friction -- the M70 didnt even have roller lifters. Reliability was greatly enhanced by the new Seimens electronic engine control system using the single CANBUS system. The engine was significantly lightened with such things as sintered hollow camshafts. It also runs much cooler due to better airflow path design and exhaust and oil cooled piston skirts. It also has the ability to take advantage of all the advancements in 100,000 mile wear items like Bosch Platinum +4 plugs, Bosch Design III injectors, synthetic Esso tranny fluid, etc etc.
There is actually a differences training manual by BMW between M70 and M73. You will find no ECM chips available for the M73 as BMW figured out what improvements others made, nor will you for the newer steptronic.
Fuel economy also improved with the much crisper shifting stepronic 5 speed 5HP-30 model. Only 363 of these caem to the US though.
If you want an 8 series, you cannot do better than the post Oct 95 850Ci.
It was discountinued for the US at end of 1997, and world wide 2 years later.
if you ever get the heads off, I highly recommend a port and polish too. You will get massively huge amount of tourqe and better milage. I get 22.5 mpg, and still make it to 60 mph from zero in 5.8 sec, but the real kicker is the 0-100 times due to the tall rear end 2.8:1. If you want an absolute rocket from 0-140 spend $800 and get a 3.15 rear diff [must be 25% LSD though].
Also if you are at all adept at working on it yourself, you MUST get the ETK and manuals on CD-ROM. This is the Falcon100 of business cars.