If you still have time to evaluate I would recommend to go straight to version 6 as it brings way more features that were lacking in previous version such as:
- improved session management
- improved timer management
- clustering support
- completely redesigned web tooling
- advanced BAM capabilities
- and more
Version 6 is targeting AS 7.1.1 and EAP 6.1 so that is the latest JBoss AS versions available and that is the recommendation if you like to run it as part of the application server.
Regarding BRMS it would be best to talk to RedHat sales to get details on that as this is the community version we talk about here.
HTH