Hi John, Lets take your questions one at a time.
Surface prep. Close joints are always good but not essential, glueing plastic involves a form of welding, ie the surfaces melt together forming a filler as well as a glue. Plastic weld is a model railway product which is very good, it's a liquid applied with a brush and the pieces are held until the liquid evaporates. Prep all surfaces as there might be release agents and / or fingerprints on the surface, washing up liquid is great for this, also when ready for painting wash again and lightly scuff the surface with fine wet and dry paper, (600 grade).
Epoxy. is not a good glue for plastic hulls and superstructures as they flex and move, epoxy is brittle and will fail over time.
Finish painting with a plastic primer, (Halfords) this gives a flexible basecote, then paint as desired, i use car type acrylic paints, if you want a colour not as a car, you can get paint made up to your spec.
In short, Liquid poly glue, clean before the build and when ready for paint.
Hope this helps
Mark