Jordan has given excellent advice in the post right above mine.
If you can afford a Leica lens, I'd go for that. Probably your best choice would be a Summicron version 3 or 4. The V4 is undisputedly one of the best 50s of all time. The V3 is usually a bit cheaper, because it has a reputation (largely undeserved) among some of being a bit "inferior" to other Leica 50s. Many of those who use it sing its praises, and because of its somewhat bad rap, it can often be found cheaper. Unlike the V4, it doesn't have a focusing tab, so you'd have to decide how much that means to you.
The V2 (rigid) is fantastic, but as Jordan mentions, it won't focus closer than 1 meter (modern Leica lenses focus down to 0.7 meters), so you might want to go with something newer.
And if you need to go cheaper--so you can buy loads of film, that's definitely a priority!--the Zeiss Planar 50/2.0 ZM has the reputation of being a genuinely stellar lens optically, if maybe not 100% quite up to the build quality of a Leica. I've seen some truly fantastic work done with that lens. And in the end, the photo is way more important than what's stencilled/engraved on the lens, so I think you'd be totally happy with the Zeiss lens, too--maybe even more so than with a Leica.