If you want to keep looking at the mini-ITX form factor Mini-ITX Tower case options
I has thought you meant that you'd be 'adding components' to the mini-ITX cases instead of adding more computers.
I'm thinking mini-ITX is still a reasonable option for you. Without a separate graphics card any PSU in the 200W-250W range should be plenty. And any of the recent dual core CPUs will be fine as well.