First, let me welcome you back into building.
There are many different answers to your questions and they come down to building style. We've all developed the ways we build and there's no wright or wrong for the most part.A lot of things do make a difference, though.
On engines I paint all the separate pieces before assembly. I do glue block halves first and heads also if they will be painted the same colors. Color is in fact what separates the sub-assemblies I might have. I try to many different metal paints and spray as much as possible. Testors Metalizer spray paints are a good start for good looking metal finishes.
I usually only use primer on the body parts because almost everything I do required bodywork. Also, I use lacquers a lot and those paints need a primer to protect the metal. The primer also insures a uniform tone to the final paint.
As far as glue, a liquid plastic glue, such as Ambroid Pro Weld (and others), a medium Ca GLUE (I like Zap-A-Gap CA+ because I use the whole bottle before it dries up), a 5-minute epoxy and a glue for windshields and clear parts ( I use a watch crystal cement but Testors has a windshield cement)
Enjoy yourself and look at all the others ways of doing things that I'm sure will get posted here. Then chose what seems right to you.