But if you do want to use an Arduino, since you want MIDI over USB in then it might be easier to use a different Arduino like the Leonardo or Pro Micro which has the USB connection already built in.
Yes but these will only act as a USB client, the OP needs a USB host to plug his keyboard into.
I think the OP wants to not use a computer but to use and Arduino to convert from the keyboard to a 5 Pin output. That can't be done without a host shield, no matter what type of Arduino he chooses.