BuddyPress.org

@whitbyglennk
That really depends on your theme. If the theme you use uses standard BP-Default theme markup (e.g. a child theme), then Buddyvents will work straight out of the box. There might be some minor CSS issues, but not more than with other plugins that use standard markup (depends on your modifications).

The payment is for version 1.x only. Basically, while the plugin version stays at 1.x all updates and support are free. Once the plugin reaches 2.x the plugin will have to be purchased again at half price for existing customers. The readme file, where you can have a look at current/future features for various versions, can be found here:
http://test.shabushabu.eu/2010/10/readme-buddyvents-dev-v1-2/