You found what you are looking for here! As for which are "the most collectible" that's kind of up to you. Most all Mauser rifles in good shape with good bores will out shoot most modern rifles. Remember these rifles were made from the best materials with true old world craftsmanship. Probably the best thing you can do is start buying books on Mauser rifles. Spend your hard earned $$ on books so you can educate yourself and avoid buying mismatched or poor condition rifles. Probably the two most available South American Mauser rifles which you can still obtain in excellent condition are the Argentinian 1909 long rifle and the Brazilian 1908 long rifle. Either of these would make a great rifle to start a collection as well as shoot very accurately. They are also very attractive rifles. I see them for sale regularly on the various on line gun auctions.
Below is a photo of what they look like. I purchased these over the last year. The Brazilian is the bottom rifle.
the following books would be a good place for you to obtain good info:
1) MAUSER MILITARY RIFLES OF THE WORLD By Robert Ball (fourth edition)
2) STANDARD CATALOG OF MILITARY FIREARMS By Phillip Peterson
3) CROWN JEWELS THE MAUSER IN SWEDEN By Dana Jones
I wouldn't limit your self to only South American Mauser rifles The Swedish Mauser rifles are one of the best made and most accurate military rifles ever made.