Requires Craft 2.5+, running php 5.5.9+
No. It's not required, but it's highly recommended.
While Charge 1.x installs will continue to work, they are no longer supported, and only critical bug fixes will be released.
All the features from 1.x are directly available in 2.x, along with many more. Upgrading is very quick and simple, and is free if you upgrade to Charge Free edition. Upgrading to Pro isn't required.