I bought an 83 300CD last year which was in reasonable shape and I spent last year working on the mechanicals which are now up to snuff and I have proceeded to work on the body this year. I found a lot of rust behind the battery box and also beside the right kick panel which was actually rusted thru to the outside. Fixed both (welded new metal) and heavily primed and sealed with urethane both areas and went on to the rest of the car. At first glance the front was in pretty good shape but once I looked closely there was a lot of rust underneath the front (holes) and the bottoms of both fenders were gone. I proceeded to go to my local recycled parts yard (Ollies Phillipston Ma.) and took off two fairly nice fenders from a 300D and had gotten previously from the guy I bought the car from a hood and hauled them home. Proceeded to strip all three and welded new front pans obtained from Ebay (NOS) and continued the process with some bondo and priming. After priming and more priming and a lot of sanding and a lot of sanding I finally edged them today with the final basecoat. I will keep everyone posted with the final installation of the fenders and hood on the car and final paint with clearcoat.

Looks like you are really doing it right. Will be anxious to see the end result.