I am working on a site where vCenters and hosts are v6.0, and the datastores are still on VMFS-3. Would the VMFS version present any issues if we looked to upgrade to 6.5? Would it be advisable to upgrade to VMFS-5 or everything still compatible with VMFS-3?
6.5 can read/write to VMFS-3 but it cannot create new ones. My recommendation is to upgrade to 6.5, create new VMFS-6 volumes, and migrate the VMs over to it. There is no in-place upgrade to VMFS-6 from either 3 or 5.