Unless you need to build something that's not ready-to-go from the store, it makes not a lot of sense (pun intended) to not buy a product from pfSense/Netgate.
If you have a special case, like, you are re-using what you have, or you need something even bigger than the biggest they have, or maybe you want something that's super cheap (but also performs like shit), then sure, making something yourself makes sense.
You can maybe shave off a few bucks when you try to duplicate what's in the store already, but you're basically saving a tiny amount of money and pfSense gets nothing. The software is free, yes, but buying "known good" hardware from the store is good for you and good for pfSense. They are not overpriced or low quality either, and the free support incidents you get to use are not a bad deal either.
I know it all sounds a bit 'buy stuff!'-advertisement-ish, but I don't work for pfSense or Netgate or ADI or whoever is involved. I'm just slightly worried about the current state of monetary support for pfSense. It's been a great open source project, and free software is all fine and dandy, but they are moving into the prosumer/pro space, and at this quality level you're going to need a commercial-sized cashflow to keep things going and grow. Of course, pfSense's community is a real thing too, and trying to 'make' people buy stuff never works out for anyone. That said, if you really like the project/software/people-spending-time-on-it, maybe get pfSense Gold, even if just for the autoconfigbackup. It's cheap compared to commercial equivalents yet supports the project.
Now back to your topic: instead of duplicating/cloning this build, you may be better off trying to set specs/budget and select parts on that. If you get a mobo where FreeBSD is known to run great on, the rest is somewhat easy. Some motherboards (hell, many!) have bad/broken firmwares and are a PITA to get a working FreeBSD bootloader/kernel working on. There is a FreeBSD HCL somewhere on their wiki you can try, but if you want to be on the safe side, just get a previous-gen board with CSM support in the EFI, then you'll have a really good chance it'll work fine.