I would prime it with whatever is recommended by the manufacturer of your topcoats - those guys are the experts !!!

I'm liking the waterborne idea - if you can spare the bucks, invest in a turbine system as the waterborne needs immaculately dry air. The nice thing about the turbine systems is they supply the volumes and pressures required by the gun - no farting around or guessing.

Hit the whole car with cleaners and degreasers - I GUARANTY you "someone" will have touched the bare metal with bare hands!