Durapox is great for industrial stuff and car floors etc but it goes hard as rock so using it as an etch on a resto type job wouldnt be ideal.
best stuff to put over bare steel is 2 pack epoxy etch, then do any filler work etc over that, then more epoxy to seal it off then use 2 pack high fill primer for filling sanding scratches etc.
1: best way to get all of that rusty primer off is a DA and a whole bunch of sanding
2: any 2 pack epoxy will be good, bog can be a personal preference but I use the upol fantastic. go to complete paints or total body shop. complete paints may be a bit more helpful seeing as you're new to paint. both in onekawa
3: I'm a panel beater in Napier, old cars/rust is my specialty. PM me if needed