Mamiya's are great cameras. I have two of them. Problem you may experience if you make the jump to LF without much experience of doing a lot of your won development and such is, it is a whole different way of photography. It is slower and more methodical. You ned to calculate out exposure. It is not as portable. I love it, but it is more of a love hate relationship. As for which camera to get rid of, I would sell the D100 and lens while the market will still pay you a decent amount for it. In a year or two it maynot be worth much. the F5 if you want to really have a kick, I would sell too. Get a smaller digigizmo that has some good optics and you will have around for 3 to 5 more years. Get a good Mf system and a tripod. Pentax makes a good 6x7 format camera with good lenses. With that size of negative, you will probably turn your back on digital for the forseeable future. Ebay has them a lot cheaper.
In the end it comes down to what you shoot like. If it is on the move and speed you want, few medium formats have that capability. A rangefinder is smaller and has more portability in MF. The MF rangefinder has it's drawbacks if you like to shoot telephoto. It is all a matter of tradeoffs and what you want to do. Most important, I would get a good tripod and a light meter and learn to sue them. forget about the center weighted metering of the 35mm and see what you can do with experimenting with exposures. You just may find that your 35mm photography improves enough that you won't want the digigizmo any longer.
This is all just the old redheads opinions.