The Pi Bar comes with everything you need to mount it.
If you end up with 125s a brake will be helpful on hills, but Coker wants way too much for that V-brake. The Nimbus 29" has Magura mounts so check out this thread for a way to mount an inexpensive V-brake on them (a V-brake plus lever and cable shouldn’t cost more than $25). Since you can mount the brake to the Pi Bar you won’t need to buy the $18 under-saddle brake lever mount.