Welcome to the forum Joey.
That's a lot of questions. I'll do my best to answer some and these are just my opinion. Put away the brush paints. They should be for detailing only. Airbrush or rattle can everything you can. If a part like an engine is in two halves, by all means glue them first. Primer is always a good idea, because it will help you spot any flaws before you lay paint. For engines, chassis, interiors I'm a big fan of Krylon rattle cans. Shoot me a PM if I can help.