Yes, if you configure one of the NAS devices so that both host can assess it and see the datastores then HA will restart VMs on the other host if one hosts fails *YOu need to setup a cluster, put the hosts inside it and enable HA)
If the NAS fails, you can recover the VMs via vSphere Replication UI, you need to manually trigger this otherwise your VMs might get recovered during a "false positive" outage)
If the VC is down though, then you will not be able to bring up the VMs on the other host with vSphere Replication because you need the VC to be online.
Solution 1) Store the VC on the other NAS and ensure you have backups
Solution 2) In the event of main NAS failing, manually bring them online (Search datastore for the replicated VMs and add to the inventory)
Solution 3) Or build another VC (licensing?) and replicate to a Replication appliance deployted to that VC, store the VC on the other NAS
Also, ensure you are using RAID on both NAS devices to help prevent any failures.
For your other question, Your VMs disks and VMs configuration filess should all be stored on the NAS. The host requires a very small amount of space to install ESXi, depending on the evrsion of ESXi it's something like 8GB for the install and some space for logs etc. Recommended option is to install to a USB stick or SD card installed nside the host
I hope this helps, anything else just ask.
vSphere Replication, quick youtube video here: https://www.youtube.com/watch?v=8lXSQUOY9eo
