It's pretty easy to build, especially with modern rims. I did up exactly what you're looking for, except for 120old. I bought the cheapest disc rims from ebay (at the time ~$30 shipped), already had the hubs, + cheap spokes and I'm in it for less than $100 for the set.

Only problem is that both my fixed gears that I ride are 1" threaded and I can't ride brakeless anymore...