Please be aware, that the TTL in the M6TTL only and exclusively means, that this adds TTL flash capability to the M6TTL.
The M6TTL anyway is very limited in all things flash (basically the same to your III).
The IIIF does not meter in any shape or form. There are no electronics in the camera and from what I remember the only thing they have in common is the sync speed of 1/50th.
The TTL and the M7 are the only analog M bodies that support TTL flash metering.
The M6TTL also adds a slightly bigger body size
The TTL top plate is 2mm taller. Other than that the bodies are identical in size and shape.
and a shutter speed dial, that turns "the wrong way".
Actually it's the right way, because it turns in the direction indicated by the meter display in the viewfinder.
It turns in the opposite way of the older meterless cameras like the M3/M2/M4 etc. I shoot a mix of M2/M4/M6ttl/M7 bodies and it's not really a big deal in practice.
It is reported, that it's meter also eats batteries faster than the M6 (and it's successor, the new MP).
A myth. I probably get close to 100 rolls from the batteries in my M6ttl. Maybe more.
I would save the money and go straight to the M6 classic.
If you get very lucky, you might source one, which has been upgraded to the MP finder specs, which should then price around a very good M6TTL, but have a superior finder.
Whichever camera you end up with I would get the upgrade. I had it to done to my TTL and M7 and it's well worth the money. I think DAG and some other shops are doing it for about $150-250.
This would be a much, much more helpful feature, than being able, to shoot with the handful of Leica TTL capable flash units.
Any flash, fitting the M6 hot shoe contacts will work in A and M mode anyway ;-)
The great thing about TTL flash support is that you don't spend your time screwing around with charts etc, trying to figure out what your settings are supposed to be.
You set the shutter to 1/50th and dial in your compensation on the flash (- 1 2/3 stop for me) and then bang away at whatever stop you want.
In very dark situations I sometimes like to shoot at 1.4 and set the compensation as low as -2 stops. I use a simple directional diffuser and bounce it off a wall or the ceiling.
This raises the ambient light level just enough to prevent all but the blackest of blacks from losing all detail and helps freeze motion a touch.
With practice the results do not look like they were taken with a flash and you can work very fast in moving situations. Without TTL metering this would not be possible, unless you had time to stop, meter and dial everything in by hand.
Personally I think it's a shame that Leica dropped TTL metering from the MP.
That was a step backwards.