You have a nice looking TD.
All 85 300D and 300SD have the 2:88 Differential, not just the Calif model.
scrape off the grease and grime off the left lower side of the Diff case and the ratio number will be stamped. It will be the last three numbers.
If you want to use Federal Manifolds, then you won`t be able to use the Trap Oxidizer that you want to keep. keep in mind, the Trap will need to be replaced every 30K miles. If you go beyond this and you start loosing power, it may be plugging up. this is a dealer replaced item paid for by MB.
If you go to a higher elevation, say 9 - 10K ft and have no exceleration, you will be riping that trap off. It chokes the engine down. been there, done that.
Check all your carpet for dampness, remove them and dry them and also make sure the floors are dry. the tar insulation pads will trap water under them and could cause the floors to rust.
I agree with letting Turbo cool before shutting down, if you have been working the engine like coming off the Hwy, or pulling up a long grade. they turn at speeds of excess of 100K RPM`s and have fire on one end and cold aire on the other side. you want to let the eng temps stabilize and the oil to cool the Turbo bearing.
Putzing around town and in stop and go traffic I wouldn`t worry too much with the cooling down.
Don`t know what the problem is with your drivers seat, the vinal or the seat springs. but if broken springs. get the seat spring box from a later w126, 86 -91. Preferably the pass seat, it has the up graded springs, and the pass side gets less use. it will fit into a W123, done it. just change the seat rails and install your padding and MB Tex.
Charlie