I ride an old Voodoo Wazoo that's basically gravel before it was a thing. It's not appreciably different than a modern canti and QR gravel bike.
My experience and gut tell me to tell you to get disc brakes. One of the major things I use that bike for is huge climbs and descents. The canti's can handle it but discs would do a better job.
However, you're talking single speed. Little chance that a one speed would get up what I'm talking about unless that one gear was 40x36 or 42. That would make for an awfully lousy bike. In a reasonable range, 40x18 or 22, you just won't be using that bike in the Rockies or Sierra's, so the brakes won't matter as much.
If you ever want to get shifting though, you just bought a brand new obsolete bike. It's kinda cool on an old bike, I'd never buy a new one that's not disc and thru axle.