If you are going to shoot people, the Sonnar clone from Nikon will be really good. It is optimized for closer focus. If you mainly want to shoot general things like landscape, the Canon 1.8 will be the best bang for the buck. If you want a modern sharp lens, the Voigtlander Skopar 2.5 is really good but it will cost you about another $100 or so I think.
The I-50 is a nice lens, but I wouldn't want it for my only lens. I would skip all the old Leica lenses unless they are optically perfect, which just will never be the case.