The 6000 controller (or M3) is end of sale:
http://www.arubanetworks.com/support-services/end-of-life/#MobilityControllers
Depending on the model of your supervisor card, the last ArubaOS version you can install is 5.0.x.x (for Supervisor I or II), or 6.4.x.x (for M3).
Aruba OS 5.0 is end-of-support; ArubaOS 6.4 will be officially supported for another 4-5 years (see exact date in the link above).
Read the release notes for the software you will be installing on any limitations. There are quite some customers still running 6.4 on M3 modules. If you have further questions, reach out to your Aruba partner or Aruba Sales team.