What Cliveh says...if you can afford both, get them at that price. The camera you get depends on the use, if you like the 50, then the M3 is the go. I have the M2 because I use a 35mm lens quite a lot and never use a 135 on a rangefinder.
Usual problems to look for are mis-aligned rangefinder or cloudy viewfinders, and check that the low speeds are ok. Otherwise these cameras are pretty bomb proof. Your Jupiter will be fine, and I presume it is a thread mount? In which case you will need a LTM to M adaptor. Cosina Voigtlander make an excellent range of lenses if you can't afford Leica lenses.