Correctly, the Rollei is scale focus - you estimate the distance and set it manually. Fine if you're often shooting either stopped down or far enough way that really accurate focus isn't crucial. I tried one briefly but usually shooting in the wide open - f/4 so I was misfocusing enough that I sold the camera.
Ralphs post is excellent and covers most of it. Building on that, here are some personal anecdotes about the cameras as I went on a big rangefinder binge over the last year.
The GIII is solid and a good compromise on size for functionality. If you want smaller and can deal with a slower lens, I'd throw the Olympus 35RC in there. The Yashica rangefinders are also excellent but many are automatic. The Retina IIa/IIIc/etc are wonderful cameras, compact and with great lenses if you can deal with manual only and quirky ergonomics in exchange for a tiny collapsed size. The FSU cameras like the FED 2 are nice but getting a working one in good condition is hit or miss. With increased shipping costs, they aren't the bargain they once were. For the price of buying a few to find a working one, you could almost pick up a Canon Leica thread mount camera and FSU lens like the Jupiter 8. I love the Minolta 7sII but picking up a working one was difficult - it took 4 tries. The Lynx 14 is wonderful but is also an SLR sized camera, as are many of the other larger rangefinders from Yashica.
There is a good chance the camera will need the light seals replaced. You'll see a lot of posts about a seller on ebay named 'interslice' who sells kits. Unfortunately he doesn't seem to be active anymore though the instructions from his kits are posted here: http://www.kyphoto.com/classics/sealreplacement.html and material is available from micro-tools. If this sounds intimidating, I'd look for a camera which already had the light seals replaced.
Over all, look for a camera in good condition. Frankly condition trumps almost everything except ergonomics with many of these cameras as most of the better known ones have wonderful lenses.