A bad engine problem is the only thing that makes a 600 SEL a more expensive proposition than a same-year 320-500 SEL. A 600 is basically an optioned-out W140 with a more powerful (and expensive) engine. I pray every night that the engine never lets go!
There is some "doubling" of expenditures because it has two air filters, two distributors, etc. As Steve Brotherton explains, you have to conceptualize two M104 (I believe) engines being bolted together. It appears you can extrapolate issues and approaches from the M104 to the 12 cyl.
There are so few around that most technicians have never worked on one. Therefore, it's helpful to find a technician who (a) is technically knowledgeable and (b) trustworthy. I am fortunate to be close enough (160 miles) to have Continental Imports/Steve help me with my 1992.
As for power, I have at least twice, maybe three times, "caught rubber" on the 1st to 2nd gear automatic shift while simply pulling into traffic with my foot into the accelerator (I wasn't trying to do anything but get up to speed as quickly as possible).
90 mph is, if I recall, 2500 rpm, so the engine really doesn't work very hard at speed. But the car is totally smooth, quiet, and rock-steady at that speed.
As for the Mercedes-BMW debate, a couple years back one of the car mags, either Car & Driver or Road & Track, did a feature on "affordable," i.e., used, 12-cylinders. It compared the XJS, the 850, and the 600 SL (not exactly apples to apples). The conclusion was that the Mercedes was/should be the most affordable in the way of upkeep as well as having the least depreciation.
Good luck and go for it!