Work directly with your Aruba SE to flesh those questions out. The main thing is you have to decide or know if, in 8.x you want to leverage the features on the Mobility Master (Clustering, NBAPI, Airmatch, etc) or not. If they are not critical to your need, then you can go with 8.x on a controller without MM. If you do want those features, you need the Mobility Manager.
but your local HPE Aruba SE can better work through those questions than here.