While you CAN do all of the above suggestions, they will all cost SOME money in terms of additional cables/software/etc., along with time for the learning curve. One alternative that I might suggest is that you go buy an inexpensive consumer-level DSLR that has video capability (or a dedicated video camera) and ensure you have the connections to use an external microphone. I suppose it comes down to what you have - more time and patience for learning how to sync different streams of audio and video, or more money to go buy an appropriate tool for what you wish to do.
I must admit, even though I play piano and drums, I'm still a little lost on MIDI, but I was under the impression that the piano acts as the trigger and you'd still need a sound source on the pc? Meaning that while every press of the key on the piano transmits digital information about the velocity, tone, sustain, etc. of the note, the computer uses that information and applies it against a sound source - meaning a stored sound file. So, you wouldn't really be getting your piano sound through midi unless your pc application has the exact sound files for your piano. At least, that is my rudimentary understanding of MIDI.