If I were you I would download the source code for very popular P2P programs like Frostwire. There is a pile of sourcecode up on sourceforge. Nothing better than seeing how everything fits together in a fully running project that's testable right away. Fully functioning code is great to learn off of.
http://sourceforge.net/projects/frostwire/
http://shareaza.sourceforge.net/
Once you see the daunting task ahead of you... you may not want to do it anymore. I only did a quick search for the source code for a couple of the more popular ones but Frostwire is probably the most used after Limewire. I have removed it off thousands of customer machines when I warn them about P2P (their kids are not downloading Linux ISO's) :)
Oooo... i just noticed the date on this. Well, I already typed this out and if you ever did start working on the project I'm sure you're STILL working on it :)
