Q:
Engineer Dungeon Explore Build?
A:
Generally speaking your best bet is grenades. They do massive DMG at long range, and aoe chill on freeze grenade can be considered one of the best (if not the best) debuff ability in the game, especially useful for PVE encounters. Plus grenades will very quickly get vuln stacks up to 25 so your entire group is doing 25% more dmg. Also use a rifle for CC when needed.
I also like to run with a healing turret, but it down then blow it up for some very heavy aoe healing on a short cool down.
I suggest getting your experiment on and see what works and what doesn’t. There doesn’t seem to be a whole lot of information since the game is so new. I wish I could get myself to 80 and help figure these things out before others get the jump on it. Good luck!