I am no pro but would recommend you go with one system,beginning to end.
I dont think etch primer is needed with epoxy,at least not the ppg dplf(direct to meal lead free) epoxy I am using.
I would go over everthing with 80 grit by hand or DA,wash with soap and water,then do the wipes for degreaser and prep.
Lots or interesting reads on the net,but I think Macysgarage.com has a nice write up about painting a front tr3 valance that covers the whole process.
and as mentioned decide if you want basecoat clearcoat or single stage urethane.These systems components, the ,filler,galze,primer will be 2 part.
I wouldnt worry to much about the open window time,as you will probably be sanding between coats anyways,although it can be an issue for hard to get spots,like underhood supports etc.
Realising I like mechanical work more than body work,
Tom