You have a (presumably wonderful) 1988 MB 420SEL with only 47k miles on it. Awesome automobile, barely broken in.

You are correct that the only upgrade from there (in the S line) would be to a low miles 1998 or '99 S420, S500, or S600. (Or to a W220, but at a much higher price). You're also correct that, if you found such a car, you'd want a Starmark warranty for the whole time you plan to drive it.

The W140's are legendary for their maintenance expenses. OTOH they are also truly magnificent automobiles when running right.

If you're really interested, you might consider a lease on a late model Starmarked W140, with warranty to or past the lease end. Then, you'll know your total cash outlay going in. Friend of mine just did a deal like that. The car is beautiful and he has nothing to worry about if the dreaded $5k A/C repair hits.