New tires and make sure your front brake works well. Nothing wrong with a coaster brake but a front brake will stop you quicker in an emergency and its always a good idea to have two independent brakes incase one fails. A coaster brake hub geared Huffy MTB will be a pretty heavy and slow bike. Good enough for a few miles, and unlikely to attract much attention from thieves but it won't be a fun ride if you have to ride very far.
As for adding the rear hub. You will need a wheel built around the approiate hub, a shifter for the hub and a cable and housing to connect the two. That should be it.
You can do as suggested and ride what you have (with some new tires) for a few months until you figure out what you really want.
Craig