Stuken,
I've had my III-f (s/n 600xxx) about a month now, and really like it. It was my first Barnack camera, and I'm really impressed with it.
My RF was a bit cloudy, so I cleaned the RF lenses as best I could from the outside, then cut a litttle disc of theatrical gel (Rosculux R -21), and slipped that into the little groove inside the window bezel on the front of the camera, to the shutter-knob side of the lens. This increases the contrast of the inset RF patch. Same effect as the OKARO rangefinder contrast filter, for 1/100 of the cost.
Slow speeds: they don't work on my III-f... when I first played with the camera, they did; now they don't - no matter what the front knob is set at, the shutter fires at 1/20. The other speeds appear to work fine. I've heard the slow-speeds work on this body, so I'm confident that a CLA will take care of that issue.
Loading: read the thread on loading a "Bottom-Loader", read it several times. Be very careful when trimming the extended leader on your film not to leave any "slits" or extra cuts, or notched sprocket holes - these can catch as you wind the film on, and could result in a jammed camera. You may want to use the "playing-card" trick ( placed between the film gate and the film) as a loading aid on your first attempts...
Lenses: Leitz optical glass and coatings are very soft and delicate (read: easily scratched/damaged). Use a blower / camel-hair brush on lens, then lens tissue / fluid, if necessary. If the lens looks hazy inside, then it will need CLA by a pro.
If the lens has cleaning marks or is uncoated, it may be prone to flare... try to get a lens hood for each lens. I would recommend find a UV filter to keep on the front of your lens, to provide mechanical protection for the Leitz glass.
I have a coated Summicron f 2 / 50mm on my III-f, and wound-up with some INCREDIBLY flared pics, due to internal floccus, cleaning marks, and no hood. Pics taken with the light coming from behind were a lot better.
For the money, it appears to be easier to buy a new / slightly used Cosina-Voightlander lens than an undamaged Leitz lens in the same focal legnth... I still want good Leitz glass for my Barnacks, but finding it may take some time.
Case - looks like yours has nice chrome / vulcanite: you may want to find a leather everready case or Luigi's half-case, to protect the body from wear / bumps while you're out shooting. That Vulcanite gets very brittle, and it deosn't take much of blow on the edge of a door or table to chip / crack the Vulcanite.
I scored a Leitz eveready on e-bay that was missing the front cover; it'll make a fine "half-case". If you do get a Leitz case, make sure it's for the post-war cameras... the earlier bodies are about 1/8" shorter - my 1950 III-F will NOT fit into the case for my 1932 II (D).
That's what I can offer, speaking as a Leitz newbie...
I first saw/handled a thread-mount Leica about 25 years ago, when I was in HS, and knew that eventually I would "have" to have one...
I grew up a "Nikon man", as my Dad had a Nikon S, and that was the first "real camera" I ever saw / shot... but Leitz are nice too !
Enjoy your Barnack !
Luddite Frank