Audio is audio and it can be freely imported/exported between all DAWs. I use 24 bit 44.1 Khz. Lots of guys go to 96. As for midi, most DAWs can import/export '.mid' files, (type 1 is multi track IIR). What you can't transfer between DAWs are the plugin settings (EQ, compression, reverb etc.) you've used in each project, although most plugin manufacturers provide the same plugin in the various DAW formats - vst, au, aax.
So, if you're using say, Cubase with any interface you please, what you can do is record your guitar and vocals into that, and just export those audio files for use in another studio's PT rig. (Make a note of the tempo!) Probably keep the audio dry so you can mix in PT, but if you add processing/effects on the guitar perhaps, you can export them as part of the audio if you wish. If you do any midi stuff for your synths, export the Cubase project as a .mid file. Pop your synth under your arm, hook it up at the other studio, import the midi and audio into PT and you're good!
Oh I found these 4 channel offerings for you. Don't have to buy at Sweetwater, though you might want to, just a helpful listing:
http://www.sweetwater.com/c695--USB_...cyI6WyI0Il19fQ
And another thing guys do is add the Behringer ADA8000 or whatever it's called as an additional 8 channel preamp to a 2 channel interface like the Audient you mentioned via an ADAT connection. Check it out. This is about the only Behringer item that gets any love round here.
Appreciate your PT issue, but if I were you I'd probably get a Steinberg UR44 as it seems to have good preamps and comes bundled with a basic Cubase that will probably start you off. No I haven't used one so you'd need to research, but it seems good on paper.