How many spaces you have left in that additional box?
I ask, because if you are running a new circuit, then you may need to run lights separate from outlets. So you might need "two breakers" instead of one.
If you have existing lights outside, you should just add the new ones to the same circuit. I say this, because in case of a short circuit with the outlets, you will still have lights at night.
Anyway, chances for a circuit breaker to go bad are rare, unless it is expose to lots of overloads, a bad connection, and so forth.
You can find circuit breakers type Z online starting at $30.00. Right now on eBay there is a 20 amp single pole double breaker for 29 bucks. You can use it to run two circuits using one space.