Welcome to the forum

I'll address issues 4 and 5:

4. Unless you are going to show the car (only reason IMO to keep the factory setup), dump the factory Becker 1432 system. After the tape player started munching tapes on a regular basis and I had intermittent speaker performance, I bit the bullet and had a whole system (new MB Quart speakers and CD changer too).

5. The factory offset on the 15-hole (6.5 X 15) alloys is ET48. If you have 8-hole Sportline wheels (7 X 15), the offset is ET44.