The question of when to buy a chain is easy--just buy it wherever is convenient and carry it with you. Carrying a tire is easy enough, too, provided you're not pushing terrain or performance limits.
Trying to squeeze that last 3-4000 km out of a chain or tire purely to save a bit of money is probably silly no matter where you are. That's especially true if you're already someplace where you know your way around, and even more so if you're headed somewhere more expensive.
On the other hand, I've done it lots of times. Usually in the end I regret waiting until the last minute. In any case, choosing Argentina instead of Chile is going to cost you, as will choosing Brazil instead of almost anyplace on the planet. The only saving grace (IMHO), is that the search for parts and supplies can be entertaining at times, and helps encourage exploration of a new city....if that's what you want.
Last note: if you're headed across to B.A., it's worth considering Mendoza for parts and repairs--smaller, simpler and friendlier than B.A., with plenty of shops all concentrated in a small area. However, still far more expensive than Chile.
Hope that's helpful.
Mark
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