I have a couple questions about priming my bus.
I was thinking of using a polyester filler like slicksand after ********** epoxy, then 2k high build. Am I on the right track with this? Or should I just 2k high build?

I don't like the polyester primers I've just seen to many jobs with cracks and failing undercoats, but it probably mostly application problems. I use it when doing something with large flat areas like step side bed panels there always in bad shape. it would probably work good for your bus one thing to think about is your going to shoot epoxy then polyester then HI build then color and clear, that's going to be a lot of film build. is your body so wavy you can't do it with just the hi build, a couple good wet coats of hi build will smooth out quite a lot. you can also block sand your epoxy. if you use the polyester don't pile it on real heavy, I think that's were the problems happen I've seen it a 1/4 " or more.
Jay D.

That's kind of where I'm confused. This is my first project. I can't see or feel any waves or low spots...but I'm sure once primed, it will reveal a lot more than I think.

here's my thoughts on this I would shoot a couple coats of epoxy then wait a day or two depending on your temperatures. then shoot a couple wet coats of some good hi build. then guide coat and block. if you cant feel any low spots or waves I don't think you need the polyester. after the first blocking you will be able to see were your at, if you have some low areas you can catch them with some polyester glazing putty. just depends on what you find after you block it.
Jay D.

That's a great plan. And that was my next question...what can I do if, after shooting 2k hi build, I find "excessive" low spots?

you can use the glazing but thin not very thick its just for minor imperfections. then coat with hi build and block again or if it lightly spot spray it with the hi build. looks like a cool project.
Jay D.