Its been my experience that the 126 chassis is an excellent choice. It lasts forever, unless they are neglected, and even so will last far longer than you would believe.
The "lurch" you mention would make me immediately suspect a failing flex joint on the driveshaft. Instead of a universal joint, imagine a rubber disc taking its place. The rear suspension is independent so that the driveshaft doesn't require a u-joint and the rubber helps absorb driveline vibration. They get old and the bolts begin to enlarge the holes in the rubber and it creates play. Simple, cheap repair.
Check and if needed, replace the timing chain. Do a major service. Have an experienced tech check it out, and that car ought to last well beyond 300K. We run 15w40 Rotella in them at the dealer, and they still look like new inside well beyond 100K.
A mercedes requires a competent and skilled MERCEDES technician. Joe Blow at jiffy lube will cause you problems. You don't take Nicole Kidman to McDonald's and you don't take a benz to an unqualified mokanik. It gets real expensive fixing someone else's mistakes. Mercedes have some rather unique systems and ways of doing things that those who don't know will quickly make worse.
Parts are readily available. Mercedes says it can get any part, for any car ever made by them. Go to the dealer, we still stock many parts for these, and should be able to get anything and everything for it. It will not be cheap, but one expects this from a mercedes also.
I work on Mercedes for a living, I drive one every day as my personal vehicle. Having been brought up on the GM and ford and other brands, worked at an independent and worked on everything under the sun, nothing compares. They are the finest in the land IMHO.