for what it's worth, the Spanish in the Andean countries of Peru, Bolivia, and Ecuador is pretty slow and easy to understand, and they are very cheap countries, good places to learn, in my opinion. I don't know about Columbia, but I suspect it will be faster and harder to understand. Chilean Spanish is way difficult. In any case, if you start north in May going south, the weather should be more on your side, except that Patagonia isn't really so great until more like November.
As for clothes, it isn't really any more cold or wet in Peru or Bolivia than in the US, despite the high altitude, based on my experience. Any clothes that work for you backpacking in the Rockies should be ok, I think, that's what worked for me. The only thing I wish is that I had had a nice light puffy (I bought my first one after getting back, partly because my buddy looked so much more comfortable in his than I was And Daisy's suggestion about hitting up the markets when you get there is not only cheap and practical, but also more fun. Better a souvenir alpaca wool hat than some boring american sold REI hat!
As for renting your gear there, well, that's a dilemma in my mind. I always brought my own for my various trips, I have always heard that it is hard to rent decent quality gear down there, something that you can rely on, so I never have. On the other hand, it is a drag to be hauling backpacking gear everywhere you go. And my trips were always only a month long, whereas it sounds like you will be traveling for quite awhile longer. But one gear note, I would NEVER suggest bringing a whisperlight international along, with the plan of using the easy-to-find kerosine: the kerosine is utterly nasty to burn. Ugh! You can use gasoline ok in the internat'l whisperlite, that worked for me the one time I did that. Otherwise, you can get fuel cannisters ok, but I would maybe try to bring my own Pocket Rocket from home, just so you don't have to find one...but that's just my perspective talking.
Most important of all, though, have a GREAT time!!! What a fun adventure! Let us know how it goes for you.