There are many taxi companies operating at EZE. Tienda León, located right after customs, is usually the most expensive, whereas Taxi Ezeiza, whose stand is next to the exit to the parking area, is also reliable and cheaper. You can pre-book Taxi Ezeiza through their website. This way they will have a car waiting for you.

Uber is an even cheaper option but it depends on how much luggage you have -- their cars tend to be on the smaller side. Uber is technically illegal in Buenos Aires, and as such there is no official pick-up point. Your driver will possibly call you (in Spanish probably) to arrange for a meeting point somewhere in the departures area.

Since you arrive next Sunday, traffic should not be a major issue. Besides it being Sunday, next weekend is an ultra-long-weekend here, as Monday and Tuesday are holidays, and many people will probably leave the city. The drive to AEP should take 45 minutes or less.