You may be surprised to hear that the old R129 can also be a practical daily driver, rather than just fair weather eye candy (as Borat says "where is option lever? show me now where is pussy magnet")

The M119 in my 91 has over 185K mi, runs like it's ready to go 24hrs at Daytona, can get 22mpg at 70+ cruise and burns no oil. As noted, the 4sp AT seems reluctant to downshift without manual override, but has proven itself smooth and rock-solid reliable. This car is also blissfully free of the ASR, hydraulic suspension, crackly wiring harnesses and advanced digital electronics that seem to plague later models.

Your odds of finding a good 500SL are better than a 300, as many more 500s were built. There is really no reason not to buy the 500 SL over the underpowered 300 models with the expensive, iffy transmissions. These cars are solid and not lightweights, but with the M119 they are fast; much like the old small-block corvettes but without the rattles.