If you're considering building your own firewall, you've started down a long path of homelabbing. I'd encourage you to start with a proper setup and allow yourself plenty of room to grow. You want your setup to be extensible, and the firewall is just the beginning.

I'd grab at least a 15U rack and a Dell poweredge R210. Throw in a gigabit nic and install OPNsense. You'll have room for your switches, NAS, UPS, etc... later.