The costs associated with live music at Disney are astronomical. They nail you on the band set-up becuase it takes hours, and then the time that they'll be performing.
Depending on what you want for a vocalist, you may be just as happy or happier sticking with the organ. If you have Bill Wells, you're in luck! He's excellent. He literally brought tears to my eyes when we had an impromptu meeting... and he's hilarious! Ask him about Grannie's walking farts to, "Jesu, Joy of Man's Desiring."
As for your reception, live music sets a completely different tone than a DJ! Be very careful on the group of music you choose, becuase you might get roped into just that sound or style. Also, I don't know how important to you your first dance is. Keep in mind that if it's a real band, it might no be the exact recording that you're used to.
Disney DOES NOT charge you fo an a cappella performance, etc., if you use outside talent. We met at an Edwin McCain concert in DTD on 12/29/2000. We were in negotiations with his rep to see if he would perform two live songs at the wedding. From Disney's end, becuase all we wanted was him, a guitar, and a microphone, it was not a huge commitment financially. From his end, they were going to charge us the same amount as though he was performing the whole reception. We figured we'd just catch the concert another time (was scheduled for the 30th anyways...). Also, we have our pre-reception musicians coming in. We were only charged the tuning fee for the piano to have them play. Be sure and ask about private talent because we were led to believe we had a number of other fees to pay from the wording on our contract.
Good luck!
xoxo