Hi again Andrey. A helpful website is Cameraquest. Click on Classic Camera Profiles. You can spend a long time reading these articles. Among them is a "buying guide" which will give you a lot of information about the features of each camera: http://www.cameraquest.com/mguide.htm
Cameraquest also sells CV lenses, and so has a lot of helpful information about them, including pictures.
Andrew Nemeng's site also has a lot of useful information: http://leica.nemeng.com/
My recommendation would be an M2. Everyone has his or her own personal preferences. I guess I'd say buy whatever model is available in good condition and fitting your price range. Only the later Ms have framelines for the 28mm lens -- I believe that starts with the M4-P, but I may be mistaken. Ms are nice and heavy and should balance a heavy lens well.
I don't think Leicas need to be overhauled on any set schedule. As for the cost of work, it depends what shape the camera is in, where you send it, and where you are located.