I use metel primer and C/A when doing PE parts... the paint doesn't 'sit' right when I don't prime the parts...Just be carefull of hazing when using the C/A.
I would suggest cleaning the parts up, gray primer, check for flaws, then white primer (or white paint) then color coat. Tamiya's cans give me near 2 models straight out of the can. 2+ when I decant it. As for primer....1 white + 1 grey can can give me up to 3 models.
HTH
Steve