I don't know what kind of epoxy primers you have on that side of the pond, but most available here are pretty good. I have heard some complaints about the PPG being a problem - lacquer thinner evidently will soften and raise it. I've used DuPont which gets hard as nails as well as a smaller company's product, SPI. Neither are affected by reducers after cure. The SPI can be easily sanded (280-320, and even to 600 if you choose) and used as the final primer without any 2K urethane. There is one thing with any paint system - if there is any rust, it will continue to oxidize - that stuff never sleeps. Some folks have used converters, but everything I've seen including my own use, it will just slow the 'cancer' down. POR-15, love it or hate it - some say it works fine, I've mixed results.

You don't need nor want any etch primer. That's old school and seldom used since 'wonder' primers (epoxy, urethane) have come available. Scuff the metal to bright with 80 down to 320 for a good tooth, then epoxy away.

Body filler, seam sealer, final paint or whatever - goes right over the epoxy.