if you read through the build thread I did it explains most of the details about the entire construction process, Use 10 or 12 gauge wire, even though it may be a little overkill its such a short distance you might as well just put something overkill in there. It never hurt anything to overspec parts.
If your cuts are good and the panels fit good just gluiing the joint should seal it off but you can still get inside and put some silicone or caulking afterwards if you put it on your finger first. There's really no need though if your joints are good.
I painted mine a semi gloss black which is more like a satin, using some american accents paint from lowes. I'm certain there is better paints out there but this stuff adhered well and looks pretty good, I didnt thin it at all when I sprayed it. You could just give it a try without thinning in your gun and see how it sprays first. I primed it once, then sanded lightly, primed again, then sanded lightly then applied a coat of black then used steel wool, then sprayed a final coat of black. I didnt put a clear coat, but it depends on what type of finish your looking for. If you want a piano black finish then its a whole different story. A painstaking story. But theres plenty of info online elsewhere about how to do that.