Two questions on primers. Any help appreciated.

[Knight72]

Hey guys, I just picked up a good 60 gallon compressor for a steal yesterday
I wanted to have the ability to paint my truck and run air tools. I've been looking at what to do to paint the frame in past posts (rattle cans, powder coat, por15, epoxy, etc.) for the last couple weeks leading up to buying the compressor. So I plan to paint the frame and eventually the truck itself in time, after practice. I am getting slightly confused on what to use where. I'm seeing epoxy on bare metal, high build attracts moisture, urethane (I'm not sure), lacquer is outdated. Then reading Eastwoods site they claim:

"Existing Finishes: Epoxy primer is perfect for paint jobs over existing lacquer or enamel finishes. The epoxy primer will create a barrier to keep topcoats from reacting to your old paint. Sand existing finish with 180 - 440 Grit and shoot 2-3 coats of epoxy to seal away the old finish."

After the wall of text my questions are simply:

1) is epoxy primer okay to use on the cab and bed of my truck over the factory paint, or should I sack up and pony out the cash to have a mobile media blaster hit it all, and epoxy prime before flash rust sets in?

2) What is urethane and high build used for? Instead of epoxy or on top of?

I'm picking up a couple cheap HF purple guns for $10 each to lay the primer. I've got a 60 gallon 14.5cfm @40 compressor with a regulator and in line moisture trap. And I'll need to buy the primer and other materials. Not sure what else I'll need yet as I'm still searching old posts.