I guess you want cheap, so one of the older untethered 22 megapixel backs is probably a good start, Hasselblad CF22, Leaf Aptus 22, Phase One P25. Maybe even a 16 megapixel square back would do like CFV-16, but there are not many of the out on the second hand market. An older 22 megapixel back should be say €2000 - €3000 on individual-to-individual second hand market, and about €4000+VAT at a dealer. Newer incarnations of 22 megapixel backs like the P25+ are significantly more expensive. The older backs can be a bit of a minefield concerning features, many have some odd behaviors which one might not be prepared for if used to more recent digital cameras. I would think the Leaf Aptus 22 is the one that feels most modern of those mentioned, notable is its low base ISO though, ISO25 while most are at ISO50.
Long shutter speeds is generally bad news, if you need long for real (many minutes or more) you need P25+ or P45+, and then you get to pay some extra, they have high second hand prices partly due to their long exposure capability. If you can do with 30 seconds the Aptus will be okay, although you need to clean up a zillion of hot pixels (which raw converter does quite well, so it is not a big issue).
I guess you need V-mount for the hasselblad. I don't know much about adapters for the GX680, but in general with adapters is that they can be hard to come by and they can be really expensive if bought from some old stock somewhere.