Pfsense can run on low powered, old hardware - as long as your got a decent NIC built in. However if you want to run more demanding services on your box, such as intrusion prevention software (snort etc) or network wide ad / tracker blocking (pfblocker) you will more ram / a faster CPU. I have seen people run pfsense in VMs too! Desired network throughout also matters. I would check out netgate’s website (company behind pfsense) and look at their hardware offerings to get a sense at what you may need. There are other great options besides pfsense too!