First and foremost, as others have said we need to get the kilt fitting properly. Others have given some good suggestions on how to accomplish that. If those options fail, you may need to see a tailor.

Second, definitely get a sporran. Besides completing the look it is a functional item. For your stated purposes, look at those of the semi-dress and dress classifications.

Third, a jacket. I would say get an Argyll as they are far more versatile than a Prince Charlie as they can be worn for both black tie and less formal events. And some proper kilt hose for anything dressy.