Mario,
I used standard Titebond III wood glue on mine. The wood will break well before the glue joint will. If your cuts aren't quite perfect, the Liquid Nails Polyurethane or the PL Premium adhesive will fill the gaps better. I disagree with the West System being worth the (significant) cost and (significant) extra effort (Mixing, etc.) I used Titebond III on all 12 of mine that I built, and I can assure you that they're quite solid!
As for finish, mine are finished with standard black semi-matte paint. They get a new coat about once a year and I think the look can't be beaten! The more beat up and repainted that they get, the better they look
. A touch up is as easy as a quick paintbrush job. Keep in mind that Line-X or Rhino is expensive and WILL get chunks out of it with any serious use, that will require a trip back to the bedliner place to get repaired. I don't think it's at all worth the extra expense in the long run.
Also, as with ANY finish, the quality of the job is in the prep. It should take you far longer to prep your cabs for paint (Bondo, Sanding, Edge Routing, etc) than it will to actually paint them. Said finishing also is crucial with a spray-on bedliner project, as it will show any flaw on the box.
I have 4 casters on the back of mine, and 14ga. steel grilles on the front, soon to be foam backed. IMO that's also worth the effort, to keep debris out of the boxes and maintain a classy appearance!
-JB