A lot of interfaces come packaged with a DAW... from memory M-Audio comes with Pro Tools, while its slightly more upmarket cousin M-Box comes with Ableton, and so on through the brands... so when you're checking out interfaces, keep in mind which DAW you'd like to work with. Or of course you can buy the interface without the DAW and match it with a DAW you acquire separately, but having them come together does make it easy for the first time. If you've got mics with XLR connections, why not get a few standard XLR cables and an interface with XLR inputs? Makes it easy.
The M-Audio interfaces, and I'm sure others, have two options for each input: XLR or 1/4" (not sure if they're TS or TRS) and a switch to select between the two, which leaves you with input options anyway. I'm not sure about the USB output on the piano and I'm interested to see what others say on the issue.
Have a look at something like the M-Audio Fast Track Ultra... it's USB 2.0 as opposed to Firewire, it's got 4 inputs which are switchable between 1/4" and XLR, and you can bundle it with Pro Tools and it should come in at around your budget.