I would buy a 1999 model. The 99's have the distinction of being the last year and a US only model as 1999 was the beginning of the 220 cars in europe. Additionally they solved a lot of problems from 97 on. The wiring harness issue is non existant in 97 and later cars and many of the other bugs were worked out of the cars. The 140's truly have a legendary build quality which is why parts are expensive. You get what you pay for though a car that is a rolling bank vault. The double paned glass and quad seal pneumatic sealing doors really make the interior seem very quiet.

Here are some of the upgrades you get in the 97 and later cars

Post 97 cars have a glass moonroof rather than a sunroof.
Later models have the absolutely wonderful bose audio system
Later models have a nicer climate control interface
Later model parts are somewhat less expensive.
Mono colored exteriors in later models (elimination of the two tone plastic side cladding)
Interiors have more modern color choices and wood choices (no oudated zebrano here)
The exterior was updated with more modern headlights and tail lights and wheels. This was an attempt to make the car look less massive and appeal to the european market.
Xenon Lighting was offered in the 97 and later cars.

I bought a 1999 S320 and its been an absolute pleasure to drive with very few problems. So far I have changed a few lightbulbs and a set of remote batteries. It gets 23.9 MPG in my mixed city highway driving. On the open road its at its best as 90mph seems as calm and quiet as 30mph.

I think the S320 is the best choice as the V8 powered cars have expensive to repair hydropneumatic suspensions that provide nothing to the cars ride quality and have poor fuel economy. The S320 has 230hp and despite the claims from V8 owners that the cars are underpowered have plenty of power to spare. Unless you need the extra ego boost of having the S500 badge on the back of your car or never outgrew street racing from high school you will absolutely love the S320.