The pre-facelift (2001-2002) version of the S600 has the problematic 5.8L, M137 version of the V-12 motor. Specifically, there's a well-known problem with the oil cooler gasket leaking like a sieve after a while. The oil first starts dripping, followed shortly by pouring, right onto the catalytic converter. Can we say, "engine compartment fire risk", boys 'n' girls?
Now, you'd think, "oh, no problem, just replace the oil cooler gasket and we're done."
Not so simple. To do that, you've got to remove *BOTH HEADS* to get to the oil cooler so you can pull it and replace that gasket. That has been reported by quite a few sources to be about a 30- to 40-hour job. You're looking at about the asking price of that car to repair that when--not if--that gasket goes bad.
You're much better off looking for a 2003-2006 S600 with the improved version of that V-12, the M275 twin turbo. Despite having twin turbos, this version of the V-12 engine has proved to be remarkably reliable. Mine certainly has. They fixed the oil cooler problem by moving it to a much mor easily replaceable external location. There are other things they did to strengthen the engine block while they were at it.
The only thing I'd purchase a pre-facelift V-12 for would be as either A.) a parts car for both an M275 (several engine components are the same) and a pre-facelift S430/S500 (the body and interior). The price would have to be so low that they'd be almost giving it away, which is about all they're worth. There's a reason the M137 only lasted for two model years, but the M275 lasted until what, 2014 or 2015, something like that?
Get the M275-equipped car if you want a V-12. Seriously.