Separate VMs in separate VLANs for private and shared storage. Makes things a bit less convenient for you, but worth it imo.
Somewhere between $1000-$2000 I imagine.
RAID is great for minimizing downtime but is never a replacement for a backup. The 3-2-1 rule mentioned below is a good thing to follow.
Never grant higher permissions than necessary, never open more ports than necessary. If your gaming server VMs have no need to communicate with your Nextcloud servers then setup deny rules in your router firewall to stop that traffic from being possible and so on.
Get a proper physical firewall device that allows you full control. I usually use opnsense or pfsense as the software OS.