It depends on what kind of hernia it actually is. Umbilical hernias develope shortly after birth to several months old and remain with the animal for life unless they are surgically fixed.
I would ask the owner what kind of hernia it is, if this doe has kidded out successfully before, and if the doe is in any apparent discomfort.
For small hernias, it's really more of an unsightly thing than a real danger.
I wouldn't personally use a herniated goat for breeding any more than I would use a double teated goat for breeding, but that's me. Hernias can be caused by a genetic weakness, but they can also be caused by injury-but unless you were there, you don't know what caused it, you only know what people tell you.
As for the vet-depends on the vet. Some will just charge you a "home visit" fee and call it good, some will charge you the call out fee, the gas mileage, time, plus the fee for the "vet check" So, call around and see what vet is willing to do what.
Seems to me, by the time you fork out the money for a vet, you could have just paid for an already healthy animal...
Just my two cents.