Restaurants will be open. Remember that the set menu´s will mostly be for Xmas eve, so this means 12/24... some places are offering Xmas day brunch, and I think those are more reasonably priced. After that, its not until NYE that you have these set menu things, and the NYE date is the most extravagant... the top places can charge up to US$700-800 per head for the full program.

To change $$$$, please use the Banco Nacion at Ezeiza, avoid all other money changers like the pest. Unless you start ordering French Champagne, a AR$2000 budget will be more than enough for what seems to be your gameplan.