Lusty Explorer may well satisfy your first two requirements. I don't believe it searches tag files at present, but the capability shouldn't be too hard to add; for files and buffers, I've used it for some years, and find it quite nice.
Emacs comes stock with good tags support; once you've built a tagfile, M-. on a tag will take you to its definition. (I don't use tags myself, although I really should, so I don't know whether Emacs can automate construction and update of a tagfile, but I strongly suspect it can; check the relevant documentation for details.)