Either metal or wood frame would be nice. A 12x12 would be super easy to build. Do you ever see those carports for sale on the side of the road? Those companies also sell their carports with sides and ends, and can even put in roll-up doors. They are fairly inexpensive and decent quality. The builders put them up in just a few hours. My dad has a 24x24 carport building, and it is nice.
If you can put up the building, whichever kind, you can go back and add insulation, wiring, or whatever as funds allow later on.