So how gonzo is this megadungeon going to be? The answer is, a little, but not very.
I expect some of the encounters with the wizards to get quite goofy. Despite all the automatons listed below, that's because I have a hard time rationalizing living beings in a gatehouse area that's been sealed off for millenia. So I came up with some robot buddies to try to steal the bones of the party from their still-living flesh. Even then, their presentation will be something along the lines of "a shambling horror of steel and bone lurches out of the shadows", nothing too sci-fi going on. The ones in good repair that the party meets will, of necessity, be a bit futuristic. As they get further into the dungeon, the critters will veer much more towards classic D&D monsters.
The campaign won't have much in the way of guns, jetpacks, etc. This is post-apocalyptic in the D&D sense - ruins have been deserted for untold ages, the world has moved on and isn't particularly looking back at a lost past, etc. Technology is largely turned to dust, and what hasn't isn't any more potent that magic items, so there's not much call for it.
Some of my magic items will be flavored as tech goodies (instead of a dagger +1, you get an equally mystical "phasic knife", which acts exactly the same). But essentially, that's just so I don't have players being complacent about the magic items they get. "A dagger +1? Give it to the henchman!" would be a typical reaction. However, "The knife is nothing but a thin band of metal bent into the outline of a blade. On the grip is a red jewel. When you depress the jewel, the blade is replaced by a field of blue light." is an unknown, and something to be treasured.