As the black argyll and waistcoat is the equivalent of the morning suit, I doubt whether other members of your congregation will be attired to the same level of formality. I would therefore suggest that you would stand out.
I would go for whichever kilt you want, tweed argyll and with/without waistcoat, and whaterever sporran/shoes you think is appropiate (doesn't matter if leathers are black or brown) As for medals, wear what your entitled to.