Ultimately only a qualified service health professional can give definitive advice in this area - it's worth talking to your local AFCO to ask them to query the Service Entry Medical Cell to enquire under what circumstance a person with this condition can be passed fit for entry.
Depends on the type of hernia. One of the most common one is the inguinal hernia (in the groin). I had mine repaired at Haslar having been diagnosed with one in basic training.
As for hiatus hernias, not sure they can operate on that. As Ninja Stoker says, the service professional should be able to give a definitive answer. I suppose it depends on how much it bothers you and any limitations or medication that you need.