Having a single server means that server becomes your point of failure and just making backups or creating a HA cluster doesn't guarantee it will work, you'd need to do monthly or quarterly fail over testing. I'd say you'd be better of simply with two serves. You can have all of your primary services AD, DNS on both (all AD roles on both systems) and DFS shares which would be replicated between the servers automatically. In this use case even if one server goes down you're still operational and you simply fix the second server (re-install stuff if you need) and you're good to go. You will need 2 server licenses and I would go with USER CALs as you'd only need one per user