I don't know where you are buying your PPG products but around here they are crazy expensive. Have you looked at SPI epoxy and primer? Reasonably priced and they ship for free. Quality is excellent.
www.southernpolyurethanes.com
Not an expert/professional but I would use the following sequence:
epoxy
body filler and slick sand (or whatever spray poly you choose)
epoxy
high build primer
sealer
I like to have my filler sealed in epoxy from both sides.
Don