I have an M3 and a 50mm Summicron 2.0 (rigid), both from the 1960s. One of the things I like about this combo is that a lot of very classic work was done with this camera and this lens. Mine is nowhere near that caliber, but it means that there are a lot of examples of what CAN be done and what to look for in my own images (not to copy, but to have a context in which to judge).
I only have the 50 and thus far I haven't really felt like I needed any other lenses. I might feel differently if I didn't really want to stick with Leica lenses that I can't afford. The camera was my father's, and I started using it in January. Sticking with one lens for now has allowed me to get a better idea of what I like and what I might look for if I were to get another lens -- I'd thought originally that I'd like go wider, but because of the way I use my M3 I now think I'd be better off with a decent 90, and I know I want lenses that are as fast as possible.
Since you've already been using a dSLR you probably have a better idea what size lenses you like. I didn't really start using my dSLR until after I'd been playing with the M3 for a while. It taught me that I don't compose well with a wide lens, or at least that I need to learn how to work with one better before I start thinking about spending money on one for the M3. They are very different cameras, though, with very unalike (for me) user experiences, and I probably wouldn't rule out any focal length (or decide on one) for the M3 unless I was able to actually borrow and try one on the M3 myself. I've been able to borrow a 90 on occasion, which is why I know that I like that length.
Also, congratulations on your impending graduation. I wish I could say that graduating made a positive impact on my lens-buying power, but it hasn't really.
