Even though this was the PO DD, you have a car with unknown maintenance done to it except for what the PO told you.
You need to through the various systems to have a new base line to start from.
Adjust the valves.
Mercedes Diesel Valve Adjustment Procedure
Check out the GP`s
Mercedes Diesel Glow Plug Repair
Pull the fuel sender and clean it. Remove the First Aid kit and it will be right under it. Use a 46mm socket to loosen it. this thread is for a W124, but the same procedure for cleaning it. More links than you need, but will help the lurkers and Peanut gallery and noobies.
DIY w/pix -- W124 fuel sender disassembly/cleaning/testing
W123 300D Fuel tank sender question
Fuel Sender 85 300D- Cleaned (Pics)
Cleaned Fuel Sender Unit on 300D
Replace the seat bottom if the Springs are broken. use a good passenger side W123 or even a w126 will swap right over. Just remove the seat tracks and back rest if a W126. The 126 spring box is the same and the 86 - 91 have an additional spring and the rubber foam pieces in the corner springs for more support.
I can`t help you with the Radio except to check the fuses for corrosion and clean the contacts. Do not use sand paper to clean the contacts. I read to use an eraser on a pencil.
I replaced mine in the 300D with an old Becker Analog, works every time. When 1/2 the owners manual is about operating the radio, I just said, "screw it" swapped it.
Charlie