Chose 96 for the 4 speed tranny, solid and reliable. When the 97 w210 came out with the 5 sp the trannies were touted as 'sealed for life' by mb and as a result many never had the tranny serviced until mb change the 'sealed for life' statement after many had 100k on them already. THe 5 sp is also electronically controlled and very subject to damage via spilled coffee on the shifter area, as this shorts one of the relays. Also EGR issues with the W210 are notorious as the recycling of the exhaust in this system clogged the cross over pipe and ultimately the intake manifold with tarry gunk. The EGR in the 96, and I believe 97, can be easily mechanically bypassed, though I would never suggest this as a viable option as it is illegal to modify a car's pollution control system. The turbo models still have the EGR issue and from what I hear are very difficult to beat without throwing codes.

Spring Perch in the front end is highly susceptible to rust and collapse, that being said if you get car from where they don't salt roads, ie warm climate, not too close to the coast, this should not be an issue. Some other minor issues like problematic rear window regulators and glowplugs sometimes getting stuck (most likely related to carbon build due to EGR issue) cars are solid and many are approaching 200-300k on engines and drives (at least the 96 4 speeds) with no significant issues a competent DIYer couldn't handle.