Welcome back to the hobby, and you can't go wrong starting off with Tamiya. As you will exect, there are as many answers to your questions as there are modellers on the site. But heres mine.
I don't and never have used a primer on armour. Even when i am adding PE or resin. But, if you want to use a primer, for the erman dark yellow, i would go for something light, like white.
There are plenty of flat's you can buy that can be used in an airbrush. I personally use a water based flat from microscale which i rather like. I believe a lot of people use something called dullcoat, i think that's right. But not sure who makes it.
A Lot of people do build the entire model and then paint. But i have no idea how. I build as much of the kit thats going to be the vehicle colour as i can, then paint. I leave road wheels, tracks, and tools off and paint those separate. Persoanlly, i would try that approach first and get used to the basics of armour building.
Hope that helps.
I am a Norfolk man and i glory in being so
On the bench: Airfix 1/72nd Harrier GR.3/Fujimi 1/72nd Ju 87D-3