Walk-around MF cameras are mostly relics that were never made to stand up to heavy, long-term use. Pro level Mamiya 645/6x7 and Bronica 645/6x6 were and are likely to be in better shape if you shop carefully. You might get lucky with an old folder but the fore-mentioned brands with interchangeable lenses and backs are simply more durable. Most are tripod cameras since they earned their keep in studios. Frankly, if you want that sort of IQ in a small form factor, then you should be looking at digital not crusty old roll film cameras. Been there but YMMV, as usual.