How interesting, to be invited to a cocktail party! The easy stuff first - yes, a gentleman would wear a suit to a formal cocktail party. As for yourself - you will want to wear something that you feel comfortable in. It really doesn't matter where you purchase your dress, just that you are comfortable in it, and that it makes you feel good. :) I would suggest a simple black dress, which can be "changed out" for different occasions by changing jewelry and accessories.
Have fun with this! :)
Blessings,
Bonnie