I think your best options are to look into Fuji's 6x4.5 rangefinders(GA645/GS645), as they come as close as possible to the experience that Mamiya 6 and 7 provide - they are lightweight, small, rangefinders with light meters and good lenses. The only problem is they are not 6x7. The GS645, in particular, would fit the bill and could be found near $400. In my opinion, most of the other options miss the mark:
Fuji 6x9 rangefinders - too heavy, no light meter, not 6x7
Fuji 6x7 folding rangefinder - too expensive
Plaubel Makina - too expensive
50s folding cameras - unreliable, sometimes no rangefinder, I've found lenses to be inconsistent and they often require shutter maintenance before use, only 6x6 or 6x9
Mamiya Press - too heavy, no light meter
Koni Omega - too heavy, no light meter
Bronica RF645 - too expensive, not 6x7
If you don't care about weight and light meter, the Fuji GW690, Mamiya Press and Koni Omegas could be good for you. But I don't feel like they really deliver anything close to what the Mamiya 7 offers.