I owned an '83 300SD many moons ago, and from your list, some of those things can be fixed quite easily. I don't remember all of the details exactly, but here are some tips so you can start to search for the fixes on this forum.
Cruise control - look into resoldering the points on the cruise control board OR exchange for a rebuilt. You can't send a resoldered one in for rebuilding, so choose one or the other.
Right rear window - likely the sliding jaw in the regulator has failed and won't hold the window up, hence the tape. I came up with a pretty cheap fix a few years ago.. search for "sliding jaw" and my username. Worked well for me and a few others here on the forum. I was told that MB window motors NEVER die and 99.9% of the time it's the regulator. That has been pretty accurate over the 5+ MB's I have owned.
Transmission - the 1-2 shift is affectionately known as the 'neck snapper shift' in these cars
If it thunks into other gears, look into it, but the 1-2 shift firmness is normal.
Fuel gauge - The gauge (E - 1/4 - 1/2 - 3/4 - F) and the "low fuel light" are separate from each other. One can work while the other fails. Pull the rear seat bottom (two latches on the front edge) and then unbolt the back of the seat. Lift it out of the car (careful you don't scratch anything with the metal edges... don't scrape up your door jambs!). You'll see a HUGE socket looking thing and you need a big socket to get it off (or vice grips). Can't remember exactly but it was somewhere between 25mm and 50mm in size. 46mm seems to stick in my mind... Once you unbolt that socket, you'll see that it's the fuel sender. Clean the wire carefully (don't break it!) and disassemble/clean/reassemble the unit. There is a small nut on the bottom to take it apart. Once done, your gauge and light will work perfectly. Likely algae or something in the tank clogged it up.. keep running clean diesel and it should be fine.