I say buy the hardware for pfsense as having it on Unraid although possible is extremely fiddly.
It also puts you at risk of losing network/internet connectivity should the server fail for any reason or should you require to update/reboot/install updates to the server etc.
I have my an AMD 5350 running my pfSense connected to a TPLink switch which connects everything from my network apart from my Unraid server which is connected to the router directly(Allows for external access if the switch fails). Everything is on a UPS so the only worry is hardware failure.
Just gives the flexibility to work on the hardware individually if required.
Hope this helps