[aaronem]

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.

[MetaCosm]

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).

[aaronem]

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.)