If you have classic cantilever brake bosses on the frame, tecktro? on ebay has some low cost/decent quality brakes. Decent quality, stop well, priced reasonable. No snob factor, and none needed on cross bike.
I think a big part is deciding on what kind of drivetrain you are going for. Downtube shifters/bar shifter/brake combos, etc. 1x or 2x front.
Then the frame itself- if it's old, you might want to consider carbon bars or fork to take the edge off front end hits. I've got a friend who had a cannondale aluminium fork. Beat him to death. I had a crosstek al fork. Beat me to death too.
Personnally, I'd get the largest tires that would clear the frame, to allow lower pressures and bigger hits. More comfy for long gravel rides.
edit- I'm also going to drop a deuce and say this- Get a plan of attack and price it out before pulling the trigger. I know... It's more fun tinkering with parts and the "hunt" for the perfect everything, but those parts will add up quick. There are a bunch of cross / gravel bikes in the $1500-2000 range that work really well on a platform designed to work together. Buy a set of carbon rims/upgraded rims, and they will work REALLY REALLY well. I tricked out an old bike with top of the line stuff- ksyrium ssc rims, xtr cassettes, etc. etc... It was the best of the best 20 yr old parts, but never still worked "quite right" as a whole. It's pretty decent, but, It did not hold a candle to the new cross bikes I tried and bought last christmas. Knowing what I know now, I wouldn't have messed with upgrading my older bike.
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