Welcome to the list. I have a 1980 300D and have worked on a friends 300TD so I have some experience on your car, although I will start by stating I'm no pro.

The vac system is troubleshot with a hand held vac pump. They are available for between $35 - 75, depending on how nice a pump you want. I've even seen the less expensive ones in Wal-Mart's automotive section. The good news here is that the diagnosis is very cheap, the bad news is that it might take a good while. You basically have to go through the system and test isolated sections until you find a) a leak, b) a failed vacuum motor, or c) the failed vacuum pump. The pumps are robust but occasionally do fail. Once you've found the problem, the repair should be obvious.

I'll leave electrical troubleshooting to the electrical guys on this list. I'm crude but effective on electrical theory and you're better off not hearing how I chase electrical problems.

The self levelling rear has only four major components: two shocks, two Nitrogen charged pressure spheres, pump, and a hydro-pneumatic compensator linked to the rear axle. The last piece is what gives the self-levelling to the system. As the rear is loaded, the shocks compress, lowering the chassis closer to the frame. The compensator senses this change and sends more pressure to the shocks until the body is back where it is supposed to be. I've always heard that unless the shocks are leaking, they're to be assumed good. The N2 spheres de-pressurize over time and are a common cause of the droopy butt look on these wagons. They are fairly simple to replace and are < $100 each through places like Fastlane. The compensator is also fairly robust, I'd start by looking at the N2 spheres.

For the tach, refer to my electrical guidance above.

For the seats, almost any of the 5 million 123 chassis in boneyards around the country have a seat that would fit your car. Look for a gas engined 123, like a 280E. They usually were put down earlier than the diesels and can be found with <100k miles on the clock. Seat wear is a function of butt time more than age, so a passenger seat spring will have less wear than a driver's seat and a low mileage car will have less wear than a higher mileage car. You can remove the seat covers and mix and match, as needed, to get what you need.

The passenger seat belt sounds like it needs a new buckle. I've never heard of anybody wearing one of those out, so I'd look in a boneyard for that too.

The really good news is that you have a car that can be brought back to life with a substantial savings by using "previously owned" parts. Obviously the N2 spheres, shocks, etc, need to be purchased new. Once freshened up, these cars hold up very nicely as they were all built with 300 k miles.

Enjoy!

someguyfromMaryland