How high of a structure can I build for growing hops
This is probably a Local Authority Having Jurisdiction (LAHJ) question - ie, your town or county or (heaven help you if subject to) homeowners association.

In my town agricultural structures (they intend barns and silos but say agricultural structures..) are exempt from height restrictions (I suppose the FAA would horn in if somebody built a silo more than 200 feet tall, and make them put a light on top of it. Hasn't been an issue yet.)

They can be built safely. I don't know if you can built one safely, only you know that, or you will eventually, anyway. 

For what it's worth, one of the simplest to build structures is also pretty functional - the "flagpole/maypole" approach where you put up a central pole and circle it with plants. Trying to replicate the commercial hopyard high-wire grid is a lot more complicated. Tettnang, Saaz, Halletaur, Fuggles here (I managed to kill a Cascade, somehow...)

Pulleys on the eaves of a two story house with the plants out in front can also work with no real structure building called for.