As for my experience, I can tell you that no cheap nor expensive hardware would lead you to the same performance as the embedded UTM appliance. I tried Endian both as VM and on bare metal. There's no chance it will ever perform fast as their UTMs.
I can suggest you to try the way of opensource (pfsense is a good choice too) but you'll have to see if it works well with your environment.
Many functions such as content filtering/antivirus on the http(s), or such as traffic shaping, mail proxy etc will become slow in a big environment. With Endian I experienced damn slow proxy with less than 30 clients!
If they have both free community supported software and enterprise grade hardware appliances, there should be a difference! Otherwise the price of 4-5-10000€ for their HW appliances won't be justified, compared to free ISO that can run on every 10 years old PC.
So to answer your question, if you want to go the cheap way use free software and HW you already have, but consider that you will have to give tests their time before putting such a solution in production.
Both distros mentioned here are multi-uplink ready. You can setup multi WAN having one NIC for the red side and handle routers through a managed switch using VLANs (supported by both distros) or you'll need one NIC per router. I suggest the first one. Then, you'll use your multiple WANs defining either a failover policy or a policy routing ruleset, or preferably both!