The thing about the Ikon ZM is that, while Zeiss will still service it, some of the parts are no longer available, even for well-known failure points like the beam splitter of the rangefinder separating. It looks like a marvelous camera and I would love to have either an Ikon ZM or a Konica Hexar RF, but one of your chief criteria is repairability and I would only trust a Leica M for that.
Building off of Slumgullion's comments about the M6, the M6TTL specifically has the shutter release system pass through the metering circuit board so whenever that goes, you don't just lose the meter but the operability of the shutter itself. The 2021 M6 has different electrical components for the meter from the original M6 and from the M6TTL that are likely to have service support for years to come. But that's much more expensive than your budget can accommodate and you said you don't care about having a meter anyway. I reiterate my recommendation for the M2.
A good friend has (maybe had at this point, I'm not sure) a Zeiss Ikon ZM. It's a lovely camera in many ways, but when his needed service, finding someone to do it was very difficult and, finally, sending it to Zeiss took almost a year to get the repair done. Also, when he put it on the market, the resale value was virtually nil. For these reasons, I'd not be inclined to buy one.
Yes, some bits for the older M6 metering circuitry are long unavailable (I remember one of my camera tech friends telling me about that about ten years ago).
I'm a little confused by your statements about the M6TTL shutter release ... What do you mean by
"the M6TTL specifically has the shutter release system pass through the metering circuit board so whenever that goes, you don't just lose the meter but the operability of the shutter itself"? As far as I'm aware, the shutter release mechanism triggers the metering circuit to turn on and off, but operates entirely mechanically otherwise. If releasing the shutter was dependent upon the metering circuitry, if you remove the battery from the camera, the shutter should be inoperative. This is not the case: I just tested. Remove the battery and the shutter release operates the shutter at every shutter setting, and the shutter times are as the setting indicates (as far as a visual inspection and auditory feedback can determine...). Is there something else that you're referring to?
I don't expect that the M6 metering circuitry, or the M6TTL circuitry, are particular fragile or destined for demise in the near future on a camera that hasn't been abused or allowed to have internal corrosion damage the electronics (typically by sitting for years with a leaky battery sitting in them, or in a humid environment).
The M6 build is akin to the M4-2 and M4-P as well as the later models ... which isn't quite the "hand made and every part shimmed to tolerance" of the M3, M2, M4 but a simpler to assemble, built-by-components approach (which was one of the reasons Leica was able to continue producing Leica M cameras at a profit in 1978...). They're all quite robust: my M4-2, from the first series M4-2 builds, is supposed to be "terrible" according to some, but the only thing wrong with mine is that the shutter brake has been slowly degrading such that 1/500 varies by .1EV across the frame and 1/1000 varies by .3EV across the frame. It's been like that for the past dozen years that I've owned it ... the viewfinder was full of sock-drawer dust when I bought it in 2012 and needed cleaning, and my tech told me the camera showed no signs of ever having been taken apart before at that point. He's the one who discovered the shutter brake wear, and I've verified it myself several times since with a shutter tester.
So... In the end, I don't know. I've owned quite a few Leica Ms since 1972, only a couple bought new, and have only very rarely had to have any service done on any of them. In my experience, they're not particularly fragile: they're robust and work for very long stretches with nothing but a little care in handling. Get one in good shape, nearly any one of them, and you have a camera that can be used for decades with little other than an occasional CLA (like every mechanical camera needs ...). And, by and large, there are enough skilled techs and spare parts around to keep any of them going for a very very long time.
G
