I wouldn't worry too much about a rear brake right away. Just build it up and ride it with front only for a few weeks. If you like it then get rear wheel and brake.

If it has a brake bridge on the rear triangle you may even be able to fit a BMX caliper in the rear. Lever pull won't be a proper match, but it'll be something, and far cheaper than new rear wheel and disc brake.

Front derailleurs are pretty cheap.