Tabasco, Yes what you said about Epoxy primer is correct (ie. you can do bodywork/smooothing with filler, etc) over it.

ALL Epoxy paints that I've seen or used are two part (2-K if you prefer that term), but I like calling two part paints as two part paints because it's more clear what one means, meaning that you add an activator, catalyst, or 'hardener' which speeds up drying and makes the paint more durable.

Urethane paints are two part paints but are not epoxy paints; Urethane paints use an 'activator' (or hardener if you will).

One can even add an urethane activator/hardener to 'straight enamel' paints which will also make that paint more durable.