Lots of info on this site about rebuilding your own seats.
Mercedes, for reasons beyond the comprehension of us mere mortals, installed oil pressure gauges unable to measure the high end of the oil pressure of their engines. They all peg.
Before doing anything else to your transmission, resolve all vacuum problems. In correct vacuum can wreak havoc on the transmission shifts. The fact that you have no door locks. indicates there are problems with your vacuum system. The climate control system is notorious for causing vacuum leaks. Mercedes should have provided Mity Vacs with these cars instead of first aid kits.

__________________
1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13