That sounds like a route/course and riding issue, not an issue necessitating disc brakes.
The deal with disc brakes right now is budget. If you can afford them on a new bike, get them. That's where things are going and it would be good to already be in that world.
There is zero wrong with rim braked tri bikes. Zero. The fastest (per some people's tests and opinions) tri and TT bikes are still rim brake.
As for which bike? I know a lot of folks use the Giant, but that's a cost thing. It routinely has tested kinda bad compared to the P5-6, Shiv, Canyon, etc.... Like, a bundle of grams of drag. The Trinity is just super popular at the price point.
One thing to consider also is what the bike will look like, or how you will integrate hydration/storage. Some bikes fit better and are more aero with add on storage than others. Some are better from the factory with their storage options.
People hate this suggestion...........but Slowtwitch.com. They know better than Bikeforums for this topic.
For a competent 70.3 rider, you're looking at 2.5hrs or so. If you squirt your nutrition into the bottles instead of eating separate (trick to save time/aero not sitting up to eat), a BTA setup with a liter and a single BTS bottle "just in case" is probably enough. If you want real food, have some unwrapped already at T1 and eat it as you kit up.
If I were newer to tri and about to do a 70.3, I'd get a nice used Cervelo P3 and then get some used Flo wheels for it and a decent cockpit/BTA bottle setup and go from there.