I agree with inetd. They are great cars WHEN MAINTAINED - especially from the get go. They are much more intolerable of shody maintenance than some other MBs. I would not put them in the same league as the 6.3s or 6.9s. They are so much better (maybe I should say "modern") than these two - don't want to start a bashing thread on this, please.
Keep in mind that many of the 140 series cars were "leases" with people not maintaining them as a typical MB enthusiast would. I can't imagine paying that kind of money for something and then allowing it to become derelict.
If you like power - go for the 500 series. If you want "fuel economy" (I almost couldn't type that one out without guffawing) then go with the inline 300 series.
My personal dream car would be the later model S500 140 series.
Unless you have a trust fund and are not faint-of-heart, stay away from the 600 series.