Unfortunately, a stock Angus has neither the hub nor the fork for a disc brake setup. The BB7 recommendation is right on though, those are great brakes. I have run them on my monutain bikes for years (actually the same pair of them, which are now five years old), although the fixed setup I'm working on is going to run an 8" hydraulic (it's going to be . . . atypical).

The tricky thing about TT levers is that they are usually sold in pairs. I had a set of these that I got very inexpensively with a coupon or something (I think I paid $35 or $40 for the levers and a bullhorn bar - Nashbar had everything "Nashbar brand" on sale). I never installed them, and ended up selling the unopened levers and bars for about what I paid for them. If you really want to do this inexpensively, I think the Bike Island kit looks like a very good option. You can always run the cross lever near the stem on the bullhorns for now and upgrade if you don't like it or when you can locate a lever on the cheap. You might need a new brake cable if you do that, which isn't expensive.

If you feel comfortable with any of your local bike shops, you can always go in and ask if they have a single time trial lever in a bin somewhere from a takeoff or salvaged bike or something. Most shops accumulate lots of interesting misfit parts that they are willing to part with for almost nothing. This strategy works for me every once in a while, and it can start conversations that can help you make friends at the shop, which in turn will make them even more likely to want to sell you oddball parts.