All there, even a .NET wrapper is available in Apple's CVS. You should go the CVS way and not the tarball download way (old sources). Doesn't compile out of the box, needs some background C++ information. Should compile out of the box with VS2003 (or even 2001?). We need VS2005 projects, so needs a bit of work (not that much).
http://developer.apple.com/networking/bonjour/
No need to reverse engineer anything and full iTunes 7 compatibility, of course. Wondering why you think that apple didn't release Bonjour yet. DAAP is just a subservice of Bonjour, btw. But you can read it all on Apple's well documented pages. And using Google, of course.
As said, it's all there. No need to reverse or reinvent the wheel. Also the "official" way makes sure that your plugin will be compatible with future e.g. iTunes versions. You should think about adding the stuff to the normal music module, not as a seperate plugin. Integration is the key to user success.