Why don't you get a single machine with lots of pcie slots, slap a hypervisor on it (like proxmox), and then spin up a router VM that does all the network options with virtual adapters? You can do any number of x86 operating environments if you wanted to know how to secure/penetrate them and the pcie slots let you attack/configure physical devices as well that may have their own peculiarities. And then when you're done, go after the hypervisor itself.
If you want to learn against specific branded stuff (which you should if you are in the industry) then I don't think that can be emulated. But before you go specifically for a firewalla that might not be in the sector where you are working, see what is out there (like I'd get whatever Cisco network device that has a similar OS to what is commonly being used right now).
Unless you are going to secure/attack flustered systems, I wouldnt go for multiple mini systems. If you figure 10 GBs per os (which is a lot for most distros) a single TB NVMe can get you 90 systems up at the same time.