Is rackable equipment a requirement for you? Do you want low power consumption? Would you prefer saving a buck and having a big box as a router, or you don’t mind paying a premium for a small form factor?
I would not recommend anything lower than a Celeron J1900 for pfSense / OPNSense (unless you really know what you are doing from the start). The Atom D525 should be fine too.
Things you need to know: packet inspection will require a better CPU (depends on the number of clients). Running other processes, like a proxy, will require more RAM. Running OpenVPN requires a CPU with AES-NI (which the Atom D525 does not have). I’m probably missing more stuff.
Also, have you thought about getting a cheaper device and flashing OpenWRT / DD-WRT or Asuswrt-Merlin on it? Saves a buck and it’s more compact, but it’s not as featureful (you can’t run a proxy or reverse proxy on it, no packet inspection etc.), but you can still run openvpn if that’s something you desire. If all you want is learn what you can do with pfSense / OPNSense, you could just run it in a VM and save even more (although I admit, it is not as attractive as having a physical box).