Joci,
Great questions. I would recommend bringing your own aspirin and the like. You can buy it down there if you forget it, but sometimes finding the stuff you want can be tricky and it's just much easier to bring your own and know that you have yourself taken care of. Plus, it's small and easy to fit into your pack.
As far as drinking water goes, the project has you covered. Both the casa comunal in Cangahua and the Hacienda have jugs of drinkable water for you to use and it is delivered on a very regular basis and will not run out. Bottled water is also cheap and easy to find at any store. A 2 liter bottle of water can cost you about 50 cents, depending on where you go. Just be sure to bring water bottles to fill up from the jugs we have, I always have 2 nalgene bottles and that gets me through the day with plenty to spare.
See you soon!
Micah