If you're on your third build and are still enjoying modeling, it's time to get an airbrush. You can't avoid getting one, so you may as well get it now.
For priming everything, that's a terrific way to start an argument here. The only thing I ever prime are car bodies, and I don't bother with it for anything else (except occasionally aluminum parts). On engines and interiors and the like, I find if your airbrush technique is decent, priming makes no difference. Others here will tell you you
must prime everything. Try it both ways and make your own conclusions.
Otherwise, get a decent Xacto style hobby knife, some sanding sticks, and one or two good quality paint brushes. Try red sable, from an art supply store. Some pointy tweezers are great, and some decent sprue cutters. There are other helpful odds and ends, but beyond the basics alot is just personal choice.