Been reselling Untangle since version 5.x....love it.
As mentioned above....quality of the hardware is important, especially motherboard chipset, and NICs. You want good hardware controller based NICs...not "winNICs". Intels rule! Some Broadcoms are sorta OK for smaller lighter setups. Used to use 3COMs a lot in my home made builds...but those are fast disappearing.
You often see people complaining in the Untangle forums about the systems locking up....you'll notice those threads often have one thing in common...cheap hardware. "Motherboard of the month club" budget systems, cheap NICs, non-Intel chipsets, etc.
I've built many "SFF" systems, or re-used lots of biz grade desktops at clients....just to get them on Untangle. Often I'll do that for our managed clients, where their budget is low and they can't swing for a proper appliance. In order to cut down on my time spent on malware calls, I'll take an old SFF desktop, slap in a 2nd NIC and a new HDD...install Untangle Lite version, and stick it in there as their router. It's worth my hour or two time it takes to do that, because it cuts down on MY time spent in future years...cleaning malware. For "managed clients" with fixed monthly plans...it's a win for me. And the client...since they have less downtime. But I'll admit...it's a ghetto setup using some SFF desktop or toaster PC...and you should have a few spares around to swap out in a hurry if needed.
For most of our clients on Untangle....we use either appliances, or 1U rack mount servers....or in a few cases, my colleague has them installed in VMWare. 1U servers like HP Proliants or Dell PowerEdges...gotta be careful of the RAID controller, if it's supported or not. But mostly...we resell NexGenAppliances hardware.
https://nexgenappliances.com/en/4-untangle-ng-firewall-appliances
I have lots of NG-100 models, some NG-50 and NG-25 units. Jim and Rob have those units custom made for Untangle with special options and firmware that supports Untangle well. Plus emergency backup...a CF card you can boot from if the hard drive goes belly up or it corrupts. Good feature to have on a firewall!
Advantages of going with appliances designed to be a firewall...is having multiple NICs. Gives you good options for VLANs and separate subnets and multiple WAN interfaces, load balancing/failover, etc. Yes you can get PCs with multiple NICs...but a 1U appliance that bolts into your switch rack...like a business grade router SHOULD...why not do it proper?
As for horsepower...depends what it will be doing. SPAM filtering puts the huge load on it. If you don't have spam filtering...you'd be surprised how large of a network a dual core Atom can handle...
We run our office on an overkill unit...an HP Proliant DL380 with dual Xeons, and multiple Intel NICs handling 10x statics...from both cable and DSL.