You can't go wrong with a set of formula or surly hubs, laced to mavic or velocity rims with double butted spokes. I would consider any wheel with those parts to be a perfectly adequate wheel for a fixed gear. You could spend a lot more on PhilWood hubs and ultra-high end bladed spokes, etc, but its not going to result in much better wheels. Building up a mid-range wheel may cost you a little more than buying a prebuilt of similar quality, but it will probably end up being a better build and last longer if you do it yourself. The extra cost will be quickly offset by the eliminating future visits to the LBS for wheel maintenance. If you put in the effort to learn to build your own, you will automatically be able to maintain your own from that point forward.
I learned to build my own a few years ago and I've built myself four different wheel sets. I have exactly the wheels that I want and because I can do my own maintenence, I've saved hundreds of dollars by eliminating LBS visits.