SOFS role will run on one of the nodes and in case of failure/maintenance it will start on the other node. So there would be some downtime, which is equal to the time service will start on the other node.
Guide you shared states, which VMs should be used for a different amount of users. As for the disk, you choose type of the virtual disk you're going to use with the VM. In the example, disk type SSD is used.
In addition, the following guide might be helpful on setting up Failover Cluster in Azure.
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