PFsense will run on just about any hardware that is still alive and functioning today.
For example, I have several installations in various ranges of performance, two are running supermicro 1U units with 2xL5420 CPU and 16 GB ram (production units in colocation DC each serving 1/2 rack nodes with countless VM's / CT's) huge overkill but dirt cheap, and have yet to bottom out on GB lines with traffic in the range of 15+TB data/month, and bursts of throughput approaching 750MBps 500MBps +/- sustained.
To an ASRock- phenom X4 home office use in front of several nodes containing various file servers, git repos and test CT's , all the way down to an age old AMD 500MHz 500MB ram serving 27 cam CCTV, several wireless repeaters and the local wired boxes. Which is a bit underpowered itself, yet flawless existence, for the most part. The latter gets a bit bound up at times, for obvious reasons.
Get creative, grab anything you have laying around, no HDD just use a thumb drive and a couple GB nics.