1) A power filter would probably work. If you can do so, check at the store to see if the filter will fit over the edge of the tank. I bought a whisper for my hex tank and ended up having to get a different one because the lip of the tank was thicker and the tank sides weren't wide enough for the
HOB to go on.
2) That would probably work pretty well.
3) I have a
UGF run by a power filter under my hex aquarium and have not had to break it down. It's still running fine. Even better would be if you set up a reverse UGF, which doesn't suck the gunk down into the bed, it pushes it up so the other filters can catch it.
The best thing about UGFs is that they introduce vertical circulation (a good thing if you've got a taller tank.