The CPU you listed for the build should have a built-in GPU. If the motherboard supports it, it should work out of the box with FreeNAS.
If that isn't an option though, just look for a cheap GPU on Amazon that supports whatever interconnect you have available, and check if it works with FreeBSD. The NVIDIA GeForce 210 works pretty well if you just need something to get a text console (it's cheap, reliable, energy efficient, and supported almost universally by modern operating systems). Just about anything that provides a basic VBE interface to the OS (which is almost every GPU you can find these days) should work though.