Hi Dan,
great equippment, as the others said!
I'd stick with the III - the last model: You can verify usage (counter), has better & more convenient mirror lock-up / mirror pre-release system. The small batteries in the body and back are quite lasting and, if appropriate, save some bulk. The III supposedly needs its dedicated remote control, though. The auto-prismfinder (not in your list) is in deed versatile but imho adds an insane amount of bulk and weight. The simple finder I found too dark - I hardly ever used it. The chimney finder remained my preferred choice.
Lenses: The combo 65-125-250 would cover your range closely, saving on one. Other than the 50, the 65 permits good movements. For portraits with the 250 you'll need the rail extensions, take the longer ones. For movements at infinity the 65 needs the short WA bellows.
The III-back allows mid-roll changes of the cartridge using a changing bag - saving again on bulk. Never had a problem of fogged film when keeping the cartrige in its box and another carton around (never tried it without).
Hand-held / monopod is feasable... a tripod, however, would be my preference. I suppose you are aware of the size and weight of the camera: Your HB is almost closer to a Minox than to a 680.
(Why I talk past-tense: My system fell prey to robbers. Other priorities before replacing it by the same.)
Wish you all the best and good light with the 680!