It's always difficult deciding what to include, and what to leave out. Two people will always give you quite different preferences, according to what they do. The modular approach is one answer to this, but clearly beyond your planned budget.
Yes to the XLR/TRS sockets, they were an ingenious solution to a problem.
BUT! Don't PCB mount, as that is inherently unreliable. There needs to be a little flexibility between the socket and the board. I work in audio hire, so believe me!
I'd say, though, that keeping the PSU external to the device is preferable... it solves so many problems and creates so few. Most manufacturers do it, and not always for reasons of cost.
I have an Audio Developments AD145 'Pico Mixer' (with PPMs!), which has an internal re-chargeable power pack, and the power supply interfaces through a four pin XLR. A little dodgy, as talkback headsets use four pin XLRs, too...!
I also have an M-Audio Delta 1010, so I agree wholeheartedly with Anahata!
Balanced outputs are desirable... not so sure about 'orrible phonos (RCA jacks across the pond!).
Are you planning for this to be in a 1U 19" rackmount?