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.
Actually, I think I have used a clone of Lusty on the Vim side and it worked nicely. Getting tags support working would be nice as well -- I really like the global tags option in CtrlP (I got this function name somewhere in an open file, take me there).
Lusty actually originated on the Vim side, and was ported to Emacs by someone who liked the functionality.
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.)
