Given the level of formality you describe in your congregation, I'd wear the kilt with a nice polo shirt, an appropriately colored pair of hose, decent shoes, and a semi-dress sporran. Personally, I dress it up a touch more, and would wear a long-sleeve white shirt and tweed jacket, no tie or waistcoat to such an informal church, but that's me.

Whatever you decide to wear with your kilt, I applaud your decision to celebrate such a momentous day in an appropriate way.