That's using a Netburst Xeon right? It's not going to be fast. I don't have much to compare it with but waaay back when I was running a P4 2.8 it was good for ~300Mbps.
I would expect that pass 400Mbps using firewall and NAT only but maybe not much more.
If your public IP from your ISP isn't static, you will have to use a dynamic DNS service to have a static FQDN, which you are able to connect to from outside.
The DDNS update should be done by the ISP router if possible. If it doesn't support that you may do it on pfSense, you can run a cron job with a short interval for that.
and more specific, this one's an i5 https://www.amazon.com/QOTOM-Q355G4-Factory-Firewall-Multi-Function-Appliance/dp/B06XNWLR3J/ref=sr_1_fkmr2_2?keywords=pfsense+firewall+black-boxes+i5&qid=1553882807&s=gateway&sr=8-2-fkmr2
This one is a much better choice!
I'm going to get up on my soap box here... keep in mind, you should try to support the pfsense open source project as much as you can. With the Qotom box you found, you're only about $60 away from the "official" Netgate SG-3100 box. I'm not trying to guilt you into a purchase, and I'm not saying you're going to get a better experience with name-brand or not, but being that close in price, I would send my money to Netgate and the firewall project itself.