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I'd definitely recommend getting one of the M adapters with a focusing helicoid, like the Hawk's adapter. The Hawk's is built quite well, and the improvement in MFD really opens up how you can use rangefinder lenses.
I got a Hawk's knock-off on ebay. It was around $35 USD, and the build is terrible. All thin aluminum, very loosey-goosey as the helicoid extends out to the point there was wobble. It was a springless friction mount, and I also made the mistake of rotating the lens the wrong way to get it off, such that the lens mount dug into the aluminum and scraped off the black paint. That said, it was really cool seeing what the ZM25/2.8 could do with close focus. The detail was ridiculous. I'm considering going all in and getting a Hawk's.