Yeah, I'd say epoxy prime, filler, epoxy prime the filler, then continue with your system. You may want to check with your supplier if black primer/filler is the way to go, especially if you're painting white on the roof. I'd go with the urethane as well. Most important IMHO is to use a wash like DX440 or DX330 before you do anything, and just before the Base coat. A couple of tack rags before Base coat wouldn't hurt either. I'm very weary of the DPLF by PPG, it's never worked very well for me. The old DP wasn't bad, but this new stuff is for the recyclers.
I usually mask off the base coats and clear the whole car in one step. That way you don't have any tape lines. Those numbers are supposed to represent coats? If they are, you should probably consult the PPG Product sheets. You may have problems with too many coats especially with the clear. I usually paint base until it covers (Omni may require a ridiculous amount of coats, deltron usually covers in two). From past postings I know there are guys on this board that put about a 1/4" of clear on (just enough to hide the body lines), but I've always found that 2-3 med. coats is enough.