I'm not a guru by any standards but my preference is the third option: The first generation Helios 44 with M39 thread. It's also the only Helios I've got. I use it with a M39->M42->PK adapter setup on my K200D, and lately it has been my most used lens.
The preset aperture is so nice to use in AV mode (though I have to remember to up the EV compensation +1.5-2 to get correct exposures). I usually leave the preset aperture ring at 16 and use the stepless aperture ring to close the aperture as needed.
Only thing I'd like to change about this lens is the focusing. The focus ring is a bit stiff on my copy and the throw is longer than on my other ~50mms so you cant get from the closest focus distance to infinty with one turn. Also the focus ring is at the mount end of the lens so that the built in flash can get in the way of my hand when focusing.
It's also the only lens that looks good on my silver K200D because it's (almost) the same color.
All I have is the M42 version but I think a K version would have been nice to avoid the hassale of the adapter. BTW I have the 44-2 and it's sharp even wide open. I got it for $30 shipped from ebay.
I use the 44K-4 because a) don't need to fiddle with the adapter and b) the 44M has a tiny spot of fungus and is on the DL until I get around to learning how to disassemble/clean lenses.
Really there's not much difference if any. All of the Helios-44X-X lenses perform similarly in my experience.
I would get the K mount for convenience, but I do play on getting some other M42 lenses like the Jupiter-9, and I don't want to only carry the J-9 around on those days that I go M42-only - I almost regret getting this genuine adapter! All my old lenses are still K-mount (M 50 1.7, Viv 28/2.8, 135/3.5)
On the other hand, these adaptalls are looking pretty nice...
I got the K-mount version and was very happy until the wife discovered the Zenit Avtomat that came with it. Now the Helios resides on the Zenit in my wifes camera bag.
I use the 44K-4 because a) don't need to fiddle with the adapter and b) the 44M has a tiny spot of fungus and is on the DL until I get around to learning how to disassemble/clean lenses.
Really there's not much difference if any. All of the Helios-44X-X lenses perform similarly in my experience.
When you figure out how to disassemble your 44M, post the steps on how it is done. I need to re-lube the focus helix on mine and am hesitant to just tear into it without some notion of what to expect.
Well, I like to lie and wait for the exact version I want - I am in no hurry.
How hard is it to modify the adapter / lens to convert a M42 to K permanently?
It is not hard. You remove the metal retention spring from the adapter and notch/drill the lens base to engage the lens release pin on the camera body. Some people even also a drop of Loktite (the non-permanent type) to the threads.