You would be best reading the Watchguard X700 thread here, all 24 pages of it!
The X500, X700, X1000 and X2500 are all identical hardware.
People have used CF and HD but CF is easier as there's no mounting hardware for a HD. There isn't a graphics card onboard, but there is a PCI slot if you have a card, so most people use serial.
The front ports are individual Realtek NICs.
They all have crypto hardware, mostly SafeNet SafeXcel-1141 which is supported by FreeBSD but there are mixed reports of actually getting it working.
Check on Ebay for the more recent X550e or X750e or the much rarer X5000, X6000 or X8000. All great hardware.
Steve