To me it sounds like the Stowbike is the best place to start since it has all its parts. The rust on the rear gears can be cleaned up and the chain replaced if needed. If you have to get new tires, get the smoothest tread that you can find to reduce rolling resistance. The ride to work will be no problem at all but the uphill ride home will be harder so the proper gearing will be important and I am not sure what gears your Stowbike has but a MTB should have decent gearing. Let’s face it, the look of the bike is not as important as the function of the bike because the important part is getting to work. So jump into a rebuild and try to determine the functionality of the bike before you start replacing parts. In other words, does it shift through the the gears up and down, do the brakes work, as you turn things do the existing bearings seem fairly smooth, it so, then you can start replacing needed parts.