Wendell has a guide that covers various aspects of setting up TrueNAS Scale, check it out. TrueNAS Scale: Ultimate Home Setup incl. Tailscale
Honestly anything will work. Look up pcpartpicker and find something that matches your requirements. Maybe look for more SATA ports.
16GB should be fine for containers and a few VMs. If you want to run lots of VMs in the future then 32GB would be better.
ZFS supports multiple datasets on one pool, and can deploy VMs as zvols on top of the pool. TrueNAS allows you to deploy ZFS through its web gui.
TrueNAS lets you manage containers and VMs through a web gui.
Personally I use rclone for this, it allows backups to most cloud storage providers. However it’s a linux CLI tool. TrueNAS may provide a builtin option that may be better for you.