Welcome to the forum. You've come to the right place for help and advice.
Brakes: The coaster brakes on your Cranbrook are okay but we urge the installation of a front wheel brake on all builds without them. As for your choice, disc brakes are far superior to any other option but they are not easily installed on a fork not equipped with caliper mounting brackets. Venice Motorbikes, a member and sponsor here, offers some options that you might want to explore. http://venicemotorbikes.com/
You other route would be to go with rim brakes. Not as good as disc but much better than just relying on the rear coaster brake.
Good luck. Ride safe and have fun.