If you are going to stick with the M3 then you would probably be better off getting a Summaron with the goggles. The goggles change the viewfinder to the appropriate field of view. A screw mount will certainly work, but you would need an external finder. The Jupiter is not a bad little lens, but it is a crapshoot to get a really good copy. The Summaron is a bit underrated IMO. I would rather have it than a Summicron. Personally, I plan on sticking with my Biogon but it is a whole different kind of lens. I went through a lot of 35s before getting it. I haven't looked back.
If you like the "older" look of the Summaron, you may want to take a look at the Canon 35mm f/1.8. I had one of those years ago when they sold for a song. It was an excellent lens stopped down a bit and has a similar look to the Summaron. I am guessing it would be less than the Summaron, but I don't follow Leica prices much anymore.
Edit: whoops, posted over Frank who said the same thing.