Making China imports expensive and cumbersome makes these builds difficult in my opinion. It is most certainly not really $300 if you have good connections, second the admin and tolls can make you spend enormous amount of time and money.

    Electronics (MCU, sensors, displays, cables, LiPo, switches): ~$125 / ~£100
    PCB share (DIY assembly; amortized per build): ~$10 / ~£6 [full 5× PCB batch ~ $35 / £28]
    Hardware/fasteners/mech bits: ~$25 / ~£20
    Optics (lenses + beam splitter): ~$115 / ~£90
    Printed parts material: ~$25 / ~£20
    Rough per-build total: ~$300 / ~£235 (add shipping/taxes and any PCB batch overhead you keep)

That's still very cheap for an interchangeable lens MF rangefinder. A Mamiya 7 will cost you around $2000 just for the body.

There's the Mamiya Press which you can get for cheap but those are very large and heavy.