2002 S430 with 90,000 miles the doors lock including the trunk doesn't work

If the rest of the car's in great shape, it's worth six grand. so why would you put eight grand into it for anything. If you can find an independent shop who'll fix this for a grand, that might be OK. Better to sell the car with this disclosed, as there are mechanics looking for cars that need simple work, or to someone who doesn't care that the doors & trunk don't lock. Then get something newer that has no defects that bother you.

If you have money to throw away.

Find an independent mechanic that works on German cars. Mercedes cars can be a money pit.