X2 on the tank sender. This is pretty common to all cars, not just MB's. If it doesn't work at all that's a different set of issues.

The no-start can be a bit more involved. Assuming it cranks but fails to start it probably glow plug related and there are threads on this topic that far exceed my humble ability to enlighten you.

On the radio - pretty much any car audio center should diagnose that for free or cheap and recommend a fix. They do this all the time and it will likely be quicker and easier for you. Of course, they want to sell you a stereo and you might choose to go that way.

Dan