When paint is sprayed, it needs to be thin enough to go through the gun. An alkyd dries so slow it invites runs so the naphtha is used as the thinner because the paint will dry a little faster and is not as prone to run. Usually what I try to do is spray the paint with thinning it at all. Today many paints are watered down and you can spray it right out of the can. It's always better to not to thin it more than necessary to spray it.
Well they say it's better to use the Rustoleum primer because they’re selling primer. Any new metal is better primed with epoxy primer. The epoxy primer will provide a barrier to rust that no other primer will provide. Other primers will allow metal to rust underneath the paint. Often you can see painted metal with rust bleeding right through the paint. This won't happen with epoxy. One thing I forgot to mention is epoxy primers have a recoat window, which means you have to topcoat within usually 72 hours or it has to be scuffed or re-primed. Also epoxy primer is not a sandable primer. If you need to sand and level the finish you will need a filler primer over the top to work it. I use Evercoat Featherfil for that purpose. It's a fiberglass filler that smells like bondo.
I can't really help you on brand of hardener. I only have access to the Valspar hardener for an alkyd. It is available at Tractor Supply Stores. I've asked for it at Sherwin Williams and they said they didn't have any. Perhaps the clerk didn't know his business, I just haven't asked again.