Cody, I'd suggest a 3 to 3.5 Amp fast blow fuse (and some spares). I'm not aware of a fast acting breaker rated for this range and would not recommend thermal breakers as they are generally considered to be too slow for modern electronics.
Like @Craftech said, adding a TVS diode would also be good in addition to the fast blow fuse.
Doing the math: 75Watts input power X ~80% efficiency = approximate Output power of 60Watts or so.
60 Watts/18 Volts = 3.33 Amps