I specifically recommend you consider specifically a PC Engines APU. http://www.pcengines.ch/apu.htm
They come with 2 or 3 gig interfaces, and have internal mini PCIe slots too. You can choose between the 2GB or 4GB ram models, unfortunately it is soldered onto the board. The mSATA cards listed are confirmed to work with pfsense.
They use a serial ...
Mini PCIe gigabit ethernet is a thing it seems.
Some hacking needed (and warranty voiding maybe?) or maybe a 3d printed lid if its a proper NUC. I'd tradeoff wifi, I suppose, but there's reasonable workarounds for that.
It might also work with my current brix, though I need to work out how to cable it though
If you like the NUC then stay with it.
A NUC or other small hardware can run pfsense well with one NIC, provided you have a managed ethernet switch, and you're prepared to have full duplex 500 Mbit through your firewall rather than full duplex gigabit.
I had a small shuttle board/case with one onboard NIC, and no provision to add anything else. USB was ...
Here's my experiment with Intel NUC:
I managed to add a second nic by cutting a hole in NUC.
(i3, 8GB, flash drive as boot device)
More details here:
It might be worth it to look into 3.5" drive enclosures. I've got an aluminium one which is a little oversized for a 3.5" drive and which has just the right size for a NUC board and some 2.5" drives (30mm height in total).
It even has a status LED (that's the cable you see) and a kind of sled onto which I can mount stuff.
This might be something you can ...
If you want a non NUC class machine, just go with a mini ITX system. They arn't as ludicrously small but they have more expansion capability.
Looking at these reviews something like the asrock 3150 or 3700 based boards might be a good fit - up to 4 Sata ports (or even 2 in the alternatives) one mini pcie slot (which might be good for 2 more) and a PCIe slot ...
Its a bit tricky. There's three elements to your problem and each of this needs to be handled seperately. Data, Power and 'space'
Easiest way? USB 3.0 Hard drives. They are typically fast enough for bulk storage these days, though clearly you arn't going to get blazing fast speeds. A 2.5 inch drive goes up to 2tb, which is pretty impressive data density, ...
Consider the Logic Supply ML100G-10.
RAM is a single SODIMM
CPU is an Intel Celeron
My company uses a lot of NUC-class machines. While I have no experience with this particular model, we haven't had any significant issues with Logic Supply products.
I can't speak to availability or how you feel about the pricing, but it is a NUC ...