As this is pretty much an informal gathering--and it will be hot as blazes-- here is what I would recommend you wear:

1. Your kilt.
2. A white cotton or linen shirt...either with a spread collar or button down collar.
3. A nice neck tie.
4. Your black sporran
5. Hose of any colour besides white and black. What tartan are you wearing?
6. Black shoes.

Simple, as cool as is possible. Forget a belt. Forget a jacket ( unless you have the time and resources to have a linen or hopsack kilt jacket made up ). Don't worry about it all, and have a grand time!