Definately go with the Prince Charlie if it's formal and you have the option, complete with bow tie and wing collar shirt. Plain off white hose would be the ticket in my opinion. The shoes I'm not sure about. Gillies are traditional with the full rig but I don't know that wing tips wouldn't work as well. Definately a dress sporran. I know there are some leather ones with the cantle sold as dress sporrans, but to my eye they don't seem quite right for formal, go with the fur. The sgian dubh would be a nice touch if it won't create a problem and I'd pass on the fly plaid, I don't think it's neccesary and would probably be overkill since it's a dinner and not a wedding.Buy the articles you expect you'll wear again if you can, but anything you have doubts about ever needing in future you'd be better off renting.