the tone from the amp is OK, could be better but nothing I can really complain about I am borrowing the thing

. I'd like to stay under the $300 range for everything.
I have a small room but if I got a multiple input interface, could I effectively record vocals and guitar at the same time? Would they leak into each other's mics? (I could throw the singer in the closet hah). If my room is too small to do this or if the mics to make this work are way expensive then I'm fine with doing like how we've been. drums & keyboard at one time, then guitar then vocals separately.