Pierre - the Audax Mk.3 is a superb frameset and seems perfectly suitable for what you want to do on it. I bought mine as a frameset and built it up with Campagnolo Centaur (but with Ultegra brakes) and have enjoyed riding it very much.
I haven't ridden mine loaded yet but can't see that being a problem.
So……ultegra rim brakes fit the mk3 frame? This is what I was planning to fit.
I can't answer for The Raucous AUK, who doesn't appear to have visited this forum for a number of years, but I suspect the reason he didn't use Campag brakes to match the groupset, is because they're standard reach and the Ultegra brakes he used are likely to be the now obsolete long drop version.
Have you measured it yet? That will be the definitive answer. I do note that the Mk3 currently of offer in the for sale section is listed with DD brakes, so I'd be surprised if standard drop fitted.
Some Audax riders who couldn't bear to put anything other than Campag on their bikes, used to file the slot to make it reach, always seemed a bit obsessive to me, but there seemed enough material left for it to be OK. I'm not recommending it!