transmission - it'll be a while before your transmission strands you. Keep up with fluid and filter changes. Maybe the rattle is worn bushings in the shift rod. Wouldn't contribute to the delay in engaging reverse.

timing chain - it's more the absence of direction changes as in a V8 that allows the M103 chain a longer life. Replacement is more based on stretch than time. With a good oil change regimen you'll need a valve job before you need a new chain. If it's close to the stretch limit, have it changed. If the engine is apart for any reason, might as well get a new chain and as many guide rails as can be replaced based on how much the engine is coming apart.

power steering - MB says not to use ATF. If it's leaking, I doubt the right fluid will stop the leak. Fix the leak (probably just a tired retun hose) then flush and fill with the right fluid.

exhaust - try www.timevalve.com. I believe they sell just the engine pipe set including the cat but you have to call and ask.

O2 sensor - a decent tech with a voltmeter can judge the health of your O2 sensor. A decent tech will have the proper tools and won't use just a voltmeter.

plug wires - these should be replaced periodically along with the distributor cap and rotor. Any good MB tech will know how to check these items.

various bearings - remove the serpentine belt and check all the pulleys. They should turn smoothly and quietly and have no axial play or radial play.

engine shocks - release one end and push and pull on them. There should be constant resistance throughout the travel.

suspension shocks - it's up to you to judge the quality of the ride. I don't buy that bounces more than once test. As long as the second bounce has less than 10% of the amplitude of the first bounce, you don't need shocks immediately.

engine mounts - maybe a tech can take measurements to see how compressed your mounts are without taking them off. Mounts are cheap. If you're going to take them off to measure them, put in new ones.

rear control arm bushings - something your car doesn't have. You have semi trailing arms with bushings and they're a PITA to replace. If your car makes noise when you get in or out, start with new rear subframe bushings and a differential mount. If they're original, new ones will make a world of difference.

You need to find a tech that understands your style and stop listening to everyone else.

Sixto
95 S420
91 300SE
87 300SDL
83 300SD