Yes, you can fit 6 bolt discs or use rim brakes. The braking surfaces are in good condition. There's a load more photos here: https://photos.app.goo.gl/rmpK4Dqrib5kRfZV8
I'm not 100% sure. When I got the bike it was set up for touring with some steel rigid forks (I'm not sure if these were by Van Nicholas, they look quite a lot like Thorn tandem forks but Van Nicholas do currently make a rigid adventure fork so who knows).
There's a review of the Zion from 2007...
I’m not sure if these are old enough to be retro but they’re a nice set of 26 inch wheels. All straight and true. Shod in period 1.95″ Michelin Country trail tyres. The rear quick release is not original. Video of them spinning nicely here: Wheels
Collection from East Bergholt (Suffolk/Essex...
A lovely Van Nicholas Zion frame made from 3AL/2.5V titanium. It’s from 2006 but has seen little use and, as you can see from the pictures, is in great condition. It takes old-school 26″ wheels (you may be able to squeeze 27.5 in but I’ve not tried that) – see my other advert for contemporary...