Would love some advice

I am in the middle of a rest mod on my 70 Dart and am getting around to the body work. The lower drivers quarter is toast, but it seems the patch panel will cover it. I started to DA the panel and the paint comes off really easily with 80 grit. It has been painted before in 2003.

The problem is I don't know where to start.

1. Do I take all the paint off in the lower quarter area to see what I am really dealing with?

I am new to this and am just not really sure the best way to tackle and in what order.

Thanks.