I agree that you shouldn't worry. Provided you are not checking in luggage at Buquebus, you should be outside the ferry terminal in minutes, as immigration is done before embarkation in Uruguay (between Argentina and Uruguay we have our own border pre-clearance). From there to EZE on a Saturday afternoon it should be less than an hour. Not sure you'll have plenty of time for relaxing at the Iounge, but your transfer looks indeed feasible.