This past thread with a bunch of photos should help:
Also talk to UDC to make sure that the components are compatible. The Shadow handle set you linked will not work on the Nimbus 29" unless you change the seat post.
You can mount rim brakes to that frame, but it looks like in the photos, either the frame doesn’t have the magura mounts anymore, or they just REALLY want to hide them in the pictures. I got that wheel a few years ago, and mine has rim brake mounts, but I would give them a call if you want to go that way and confirm. Item descriptions don’t get updated properly sometimes on the site.
Also, if you’re going the d-brake route, call and make sure the holes for the bearing cup line up. The d-brake thing has slots instead of straight holes, but the aluminum frames and the steel frame bearing cup mounting holes don’t line up.
If you’re using it straight for off-road and you don’t really want the handlebar, there are other options. You can get just a bracket that goes under the saddle.
Maguras are VERY expensive off the shelf, if you go that route, definitely look for something used. (on this site or otherwise)