I think the fist thing I might do is buy a load more drives and not use RAID 5 anymore.... There is plenty of information online why it is not a great choice especially with hard disks. You are using real Enterprise drives, aren't you?????
Promox will want to use hardware RAID, not Soft RAID.
So yes it can install on your single 500GB disk - add another drive and make a mirror??? You can then mount the other drives at a mount point eg /var/lib/vz1 Create your SME VM and restore.
If you really want to make the most of Proxmox then use the LVM thin volumes etc but you will have to create that yourself - you will need to read about that.
You might be better getting a few more disks, make a bigger array, and put Proxmox and the VMs all on one array.
It will then sort out the thin volumes etc for you.
You can then create your new Koozali VM and import all your data from your backup.
It should recover all your settings and data - if you have not done any odd bits with your SME...... make sure you take some extra copies of places like /opt
PHPKI - as you may read in the Contribs forum I suggest you take the opportunity to upgrade to the new version (you cannot 'migrate'/'upgrade' your certificates) due to MD5 and OpenVPN.
Yes, you can decide the network interfaces that SME uses and people can access things as normal. You set up a bridge per network device and then one or more VMs can use that bridge. You can devote one network device/bridge to JUST one SME network port if you want.
On mine I have a couple of VMs that have dedicated ports, and a few with shared ones. It is very flexible.
Prox has its own firewall system that you can use if you want too. I have only just started using it
Prox does NOT have to be connected to the internet - it just depends how you choose to wire it.
It uses a bridge network interface so it is isn't 'in front' or 'behind' SME - but underneath it. You could choose to not give that interface internet connectivity and make it 'local' only.
Yes Koozali SME can be EXACTLY as it is now. The other VMs will reside on Prox as VMs in their own right (which is really handy).
Your other VMs will ONLY be exposed if you CHOOSE to expose them. It is as simple as that. I have VMs that have their own interface and reside ONLY on the local network.
Security is just the same as normal - just think about routes and firewalls.
The BEST thing to do is grab an old box, put in 2 or more network cards, and test. You will learn far more that way.