The Nimbus 29" frame has Magura mounts on it so you have a few options:
The easiest but most expensive way is to buy Magura clamps ($18 from UDC) and add a Magura brake ($110 from UDC, ~$90 from Brycer).
The cheapest way would be to drill a hole in the fork crown and mount a BMX caliper brake (~$15 for brake, cable, and lever). This would probably limit the width of tires you could run if you decided to change to a knobby off-road tire.
With about $6 in hardware you can make an adapter to install a V-brake (~$15-25 for brake, cable, and lever) on the Magura mounts. This costs a little more than the BMX caliper brake route and requires some relatively simple fiddling but doesn’t require drilling a hole in the frame and is way less expensive than using a Magura brake. I went this route on my KH29 and if you use a V-brake that advertises fender clearance (these have longer arms), the brake will work with any tire that will fit in your frame.