Congrat's on the up and comming wedding.

Matt is correct in every detail. You don't have to have a white rabbit Dress sporran Grey seal or some other type of fur is nice there are many to choose from (How I envy you) They are however expensive so start saving.
I wouls drop the belt if wearing a Prince Charlie.

You can use an evening belt I just think they don't look right .
As for underwear I use it under the kilt only if there are children around.