As for the convertible top. I used to have a 1974 450SL and when I got it similar problems with the windows. What I found out it is best to bite the bullet and just put a new top with windows. You might save a few dollars if you take the old one off and clean the frame off and/or find a replacement in the internet. Also, stick with the OEM type of material.

Add some new wheels if you do not have, and it will look 20 years younger

Kurt