The hiring of gear in Mendoza works well - it's plentiful and functional. But it can be an expensive exercise. We spent US$1500 or so for 4 of us, but we hired quite a bit. For instance - US$5 a day for an item is not bad, say, for a sleeping bag. But over 20 days, that works out to $100. And that's just 1 item. Plus there was a 10% surcharge for using any form of credit card (aside from bank fees).
Take lots of $US on the mountain. You can use Argentinian Pesos. We didn't appreciate just how much you might need. It is better to take more than you think you might need. You probably will use porters from base camp, even if it's a shared carry for the entire group (say 20kg of food from base camp to camp 1). You might have every intention of carrying your own gear all the way, and thinking you won't need the money. But you might get up there and find you need a porter from say camp 2 to camp 3. If you haven't brought money with you for that unexpected contingency, you can't use a credit card or access an ATM. So hang on to your intention to carry all your own gear, but take additional cash (say - seriously - upwards of US$1,000.00) in case it doesn't work out. You don't want to not make the summit because you needed a porter, didn't have the cash to cover it, and wore yourself out. Focus on the final objective and be flexible with contingencies.
Another example, if you get up to base camp, things screw up, and you want to chopper out - that's say US$1,500.00, cash, in advance. Also, you might might like a hot shower after all, use the internet or sat phone, or buy goods there - it's all cash based. Due to the weather, we unexpectedly went on a pub crawl up at base camp (there are 3 different bars). You might think (as we did) - "I'll never do that"; but if you haven't got enough cash, you can't go and buy a hamburger from the restaurant at base camp if the desire arises (eg, others from the group decide to).
Take tha cash, and if you don't use it, big deal. If you need it and didn't bring it with you, you're screwed. With hindsight, I'd take at least US$1,750.00 cash with me to base camp, for porters and other contingencies (another example - buying gear from someone coming back down who doesn't want to carry it), and hope I don't need it.