I also had no problem with a mid-morning check-in. (Since the economic semi-collapse the big-name hotels in Buenos Aires are rarely that full, so there should be little trouble finding a free room for you.)
Yep, the currency conversion thing can be a gotcha. This Marriott, like most of the 4- and 5-star hotels in BA, quotes its prices in US dollars but by Argentine law must charge you in pesos. The rate I got for my stay about 6 months ago was, I believe, a little bit better than the official rate but not as good as the rates most stores were using.