I'll toss in my $.02 as well. I owned an '84 300D sedan and currently have an '87 300TD wagon, so I've had all these problems, just spread across two cars...

I find all the advice given previously to be quite sound. I'll just add some elaborations and clarifications.

Does your central locking system work after the car has been parked for more than a few minutes? If so, you probably don't have a vacuum issue with the rear tailgate, but some other problem causes it not to lock/unlock. When the vacuum motors fail, they generally leak such that five minutes after the engine has been shut off, the central locking system is inoperative - the vacuum reservoir is drained. I'd disassemble the interior trim on the tailgate and look for a problem in the mechanical connection between the vacuum motor and the locking mechanism.

BTW, you get good at troubleshooting the vacuum portion of the lock system. I sold my old sedan to a friend. He brought it over for a few repairs last week, one of which was an inoperative central locking system. Took me 10 minutes to run through the system with my mity-vac and identify the problem as a failed actuator for the gas flap.

Rear suspension - you got a problem. Does the car come up to normal height when driven, but sinks back down later? The rear struts provide both the shock absorber action and the self leveling. The control valve admits additional oil - under high pressure - to the accumulators as the rear suspension is loaded down. The additional oil increases the pressure in the rear strut/accumulator system, causing the struts to raise the back of the car. There is also a minimum pressure below which the control valve will never permit the system to fall. This minimum pressure is necessary to get adequate damping out of the struts. Without it, the rear end drags, and you effectively don't have any shock absorbers.

Look under the rear of the car - way under - and you can find the control valve. It's silver, about 3" across, and has a small arm on it. The arm is attached to the rear sway bar via a linkage. (The sway bar rotates as the rear suspension is loaded and unloaded, moving the arm up and down.) Disconnect the linkage and - with the engine running - move the arm up. The rear of the car should raise. Lower the arm, and the rear comes back down. I use this trick to work under the back of my wagon - it's faster and easier than jacking it up. Of course, insure there is oil in the reservoir under the hood. BTW, the system requires a special mineral oil. It's absurdly priced at the dealer, but can be ordered for ~$10/liter.

BTW, the struts for the rear suspension are quite expensive - I've generally seen $275/each. I bought a new control valve for my 124 wagon - I bet it's exactly the same as yours - for $140. Lemme know if you need the name of the vendor. The nitrogen sphere accumulators run $80-$90 each. It's possible to get the accumulators recharged with nitrogen - this is normally what they do over in the Citroen world. Of course, you can't drive the car while they are shipped out to the Citroen specialist. If you're a real cheapskate, perhaps source a pair from a junkyard and send them out to be recharged. I think pricing is in the vicinity of $12/each for recharging. I've got a contact or two filed away if you need them.

Tailgate struts. Good pricing is about $45/each, though they are occasionally seen on Ebay for less. Just make certain you prop the hatch open so it doesn't slam shut when the first strut is removed. (No, I don't know this from experience.)

Seat springs: Just buy a new seat spring base. You can install your present upholstery on it. About 2 hours work, pretty durn easy job. To do the job correctly, you should also replace the rubberized horsehair pad. However, I cheaped out and got some upholstery foam from the local fabric store. (You'll be the only guy in there.) The spring base costs about $100 mail order. Makes a huge difference in how much you like the car.

'Nuff rambling for now.