"First plans are going to be chrome fender trim (cover rust spot on RF fender lip) and to put in a four channel amp and rear speakers."
Suggest reading through 123 posts in the Car Audio forum. You are very limited in F&R speakers due to size. Also, if you are thinking sub(s), the fuel tank sits between the trunk and the back seat, so much is lost. People have done some neat things with the First Aid Kit hole, etc. I have given up on quality sound in the 123 benzes - besides , it's nice to listen to the engine work on these cars. You can get to adequate sound, but not great. If I want truly good audio I drive the TL, with Sirius, amp, sub and stock Bose speakers inside - that is nice. I'm just always fighting the kids over which Sirius station to listen to