Hi,
You can look for something like HTPC (home theater PC) + OpenMediaVault (it is Debian + WebGUI)
OR try to installl DD-WRT on network router, for firewall only it is best option i think (DD-WRT is also based on Debian so in google you can find option to install firewall on it)
With Debian you can install whatever you want. Below you have 3 nice mini PC (below 100$ on aftermarket)
have you hear about https://www.dd-wrt.com/site/index
ASROCK ION D525 (Atom 2 cores)
ASUS EeeBox AMD E-350
Acer Veriton N282G (Atom 1 core)