What is your current setup look like? One or Two controllers? Setup mode?
To upgrade to 8.x you can use the migration tool or re-configure your controller manually. Important thing to remember is that your controllers is factory reset the configuration after upgrade.
For use all new features like clustering, live-upgrades, airmatch, etc. you need to buy a Mobility Master. What is strongly recommended for configuration hierarchy and simplify management. But is not required.
Allmost all documentation is based on a Mobility Master. There are a lot of topic about this subject on the community. If your new into this best work with your local HPE/Aruba partner.