For the ultimate protection, OPNSense runs well on even the entry-level Raspberry Pi 4B or 5. So, you can stretch your budget and add more stuff to your network without spending a fortune.
As a minimum guideline, two recommended models are the Raspberry Pi 3B+ and the Raspberry Pi 4B (with 2 GB of RAM or more). Choosing earlier models is likely to present issues with video output ...
Nilseuropa has revitalized an old Toshiba T1000 with a Raspberry Pi 4 and packed it full of handy upgrades that lend respect ...