Let's say I buy a jon boat with no motor.  Also, lets assume that I get a non-tiller motor.  How difficult is it to install a simple console, steering wheel, throttle lever, and wiring?

Depends how good you are with your hands.

Nothing major involved if that's the question.  Telestar makes complete systems and parts to build your own (steering parts)

biggest concern would be the weight added.  I wouldn't bother if boat is under 16 ft. personally but it's definately do-able if you have basic skills and tools.