Here is somebody ising Lenovo TS140 bare bones for a PFSense build:
Seems like a good option, if you do not mind the size.
TS140 are rock solid and are a good choice for standard workstation or micro server in Mid-size tower but if it need to be in a rack … well, you will lost a lot of space (4u maybe 5).
if ou have a little more budget (not quite much!), take a look at the RS140. it is like a TS140 in a 1U rackmount machine with 2 (or 3, the third is for remote management) integrated Intel NICs. It is much elegant and less lost of space then putting a TS140 in the rack ;)
I'm awaiting my supplyier to get one for testing since it is a new product.