I bought a barely used Z5 two weeks ago for my M42s and manual K-mount lenses. Iīve been using a K&F Concept K-mount to Z adapter with and without the Pentax M42 to K-mount adapter. Two dumb M42 adapters are coming from China someday..
Itīs still early days but so far Iīm happy I chose Nikon. I considered Pana S5 too, but I didnīt need all those video features and the EVF was smaller. Didnīt go for used Z6 because with Z5 I could use my old SD cards. Z5 EVF is big and seems good enough. S1 EVF would be even better but the body is just too big and heavy for small lenses.. Only Sony that was on my radar was A7RIII but was just too expensive.. And I think 24MP is great for old lenses.
IQ is really good, I think itīs a fine sensor up to 25600 ISO. YMMV.
I set up a user mode (U3) for vintage lenses, M mode with auto ISO. AF-on button is 100% magnification and Fn2 is 200%. Rec button changes lens focal length for IBIS, which works great, almost 5 stops. About a stop better than I get with K-1.
One thing I donīt like.. Focal length is not recorded to Exif.. unless you use the FTZ adapter..
. So it only works with Nikon lenses. Not sure if Pana or Sony does this either?