It's going to depend on how shiny you want things as to which gloss you use (gloss, semi, or flat). If you aren't sure what they look like, go to your local hobby shop and ask if they can show you the difference. If not, go look in the paint section of a hardware store at the gloss, semi-gloss, and flat (ignore satin and eggshell), there will be little pieces somewhere that show the sheen of each.
Do all of your painting first. I am guessing the detail lines are paint and not decals, yes? If so, paint those too. Also, are the lines recessed or raised. If raised, I like a toothpick whittled to a wedge-like point and then just a tiny bit of paint at a time. If they are recessed, I'll cover that later.
When you are finished painting, coat with a gloss coat until the finish is shiny and smooth.
Then apply decals (you likely want to use some type of solvent like micro sol and micro set, or solvaset, etc. These will help the decals settle over bumps and angles, etc. Be careful though as some are way too strong for certain decals).
Once the decals are on, it's time for a wash if your detail lines are recessed. A wash is some type of paint that has been thinned a lot. Apply this over a gloss coat so it runs into the detail lines better. You will have overspill, where you don't want it, so do small sections and wipe up the excess pretty quickly.
After all that, apply another coat of gloss to seal in the decals. Then apply your desired final sheen (if you want semi-gloss or flat, otherwise, you're done).
I don't have a good answer on the metallic paints for you. I generally use foils. I would suggest testing metallic paints with different sheen of clear coat on pieces of sprue right as you get started and letting them sit for the duration of the project to see how they do. Also do one with no clear coat over it.
Sorry for being a bit long.
Groot