I'd stay away from the twin turbo V12 cars. That's been my thinking since I bought my S55 over 12 years ago. TTV12 is just a stretch too far in terms of engineering to be reliable.
From the driving style you've described, I think the S55 is the best that can be had. I drove mine to death the first couple years, figure 20k miles a year, long hauls from MI to FL to DC to NY to MI to NoLA... a great time was had by all. Then I bought the M3 and E5E and spent less time in MI, there the car basically did summer golf course duty until a few years ago (when my daughter decided to attend my alma mater in Ann Arbor). So these days the car takes two years to cover 5k mile oil changes.
I spend about $1k per year, but it’s spread out, like $5k every 5 years — every now and then I’ll dump a few grand into it, mostly preventative and optional (e.g., new steering wheel buttons, light switches) then you can just drive it for the next few years with nothing but oil, coolant and brake fluid changes. In the last 18 months it took new ABC fluid, accumulators and tires to get to that amount — all done “before there’s a problem” which is the key point. You have to stay ahead on these cars... BreakFix is a hill you don’t want to climb.
Running costs have been such that I've never paid much attention. I cannot recall any major repair. Fluid changes are what they are but you can't go by the book because both ABC and transmission fluid were NSI -- No Service Interval, from MB when new, which is complete bollocks. Change them both every 5/50 with spark plugs and you'll be fine. Accumulators are accumulators, do them when you buy the car just to make sure, or at 100k miles. But at 15years, my advice is do them unless you can show they've been done. Maybe the ABC fluid, filter, accumulators, transmission fluid and filter cost you $2500 at a shop, so take that off your purchase price (easy to do, given how terrified people are of these cars). Engine and trans mounts are like E5E... they must be done and they must be OEM otherwise you'll do them again. But other than that, no issue. Pulleys and belts go but not often and they're not expensive. The idler pulley will vocalize its unhappiness so you’re not caught off guard, and I drove with a ruined supercharger pulley for at least 3 months before I knew it was gone. New pulleys are like it never happened. Buy one that has been garage kept -- stay away if not. There are tons of pneumatic workings in these cars so seals everywhere should not have been heat cycled, sun hardened or cold shrunk (e.g., dynamic seats, soft close doors and trunk).
If it wasn't for the buggeyed lights I'd be recommending the CL55. He should also think about the CL550 from '08-14 but the latter ones are in a different price range. Sounds like he wants sub-$15k. The only other cars I would suggest are the Audi S8 and S6 with the Lambo derived V10 engines, from '09-'10. Those can be had for an absolute steal but owners tend not to let them go. The BMW 535xi in the right spec from the same period are also good cars, but then you've got turbochargers to deal with (complexity and maintenance -- more stuff to break). My view is if you're dealing with turbochargers they need to be (1) in a Porsche or (2) with a V12 65 AMG car.
Hope this diatribe helps. Cheers,
maw
PS... I guess I should add the D2 chassis Audi S8 as well... but then you're looking more toward a restoration project... rewarding car but a bit of a headache, especially to get BT or Navi retrofitted. For that general reason I say stay newer than '06 in all of these cars.
