If you are going to get a ac->dc, get a expensive, hi-quality one.
i dont think you got time to get all the parts to make a dc-dc psu, but if youre somewhat skilled you might get this one working:
Finished Tutorial for PressLab's DC-DC
to find out how much juice the pc pulls i think the best way is to check each part and see how much its rated... if youre lucky, it might be there on every part
edit: i didnt answer both Q's