I think OP was going to virtualize FreeNAS in ESXi, not use FreeNAS for virtualization. That said, I agree with you in that I recommend against both.
Agreed, with the caveat that old servers are loud and power hungry. If you don’t have a garage, basement or well-ventilated closet to put it in, you will probably want to do a fan mod to make it quieter. Old hardware one way or the other is going to be a great value to you though. Just don’t go older than Xeon 5600s. You’ll want to have AES-NI for some future-proofness.
This is somewhat contested. If you’re doing mainly sequential workloads and have dedup off, then you can drive a very large array with 32GB of RAM. Also, to clarify, ECC DDR4 is overkill for most things, but ECC in DDR3 is more important, so if you go with older hardware, spring for ECC. It’s not much more expensive anyway.
My overall recommendation is that you run FreeNAS on used hardware with some plugins/jails and run pfsense on separate low-power hardware. My rule of thumb is that if you can separate anything physically, separate network from services.
Imo, a pfsense thin client is probably a good match for you. Here is more info on that.