Shimano Nexus cannot touch most of the more common Sturmey-Archer 3-speeds in regards to parts availability, serviceability, shifter options, etc. Plus, Sturmey-Archer offers the options of pulleys and indicator chains look rad and old-timey, whereas Nexus offers gripshifts and click-boxes--a lot of plastic, and a lot of 1990s aesthetics. Shimano Nexus hubs work pretty reliably, and they're decent hubs, but they really don't offer any advantage over the competition.
It seems like you really want to have some sort of tractor-style shifter for your bike, which would definitely look cool-- and if that's the case, the gripshifter might be the easiest to use as a basis for a fabbed-up shifter. I kinda feel like a 2-speed kickback or auto might suit your project well, with a really low direct gear and a 1.38 overdrive, and no worries about fabricating a shifter and running cables. But, in the end, you gotta do what you want with these things...