Portable copy of Win 7 to be used every day. Does your Windows license cover that usage? Seriously, I'm not a licensing guru so I'm just asking the question just in case you haven't considered i the possible legal issues.
I had a rescue disk that had a stripped down version of XP that ran from a CD so I'm sure a USB drive would be sufficient as a once in a while rescue disk solution. Sure, USB 3.0 because why not.