Welcome to the Q&A Forum

First of all I really suggest you read this post by Aleyda Solis in the SEERInteractive Blog, which practically answers to all your doubts in a very extensive way. Second, the subfolder option seems the best one for your case, at least from what I understood about your businesses' needs. In fact, business's objective in middle/long term should determine what option for International SEO you should choose, and not simply the mere SEO aspect. So, if you choose to the subfolder option, you must: create the subfolders (obvious) geotarget them in Google Webmaster Tools implement the use of rel="alternate" hreflang markup, especially because you are using the same content in different subfolders, hence, if you want to be really sure that Google shows the correct URLs for the given country, better to use the hreflang. even if you do 2) and 3) I strongly suggest you to localize the content of your site. The Spanish spoken in Mexico, Argentina and Spain is quite different, as it is the English spoken in UK and USA. The more the language used is fitting the culture of the countries you are targeting, the better not only in SEO terms, but also in the conversion potentialities ones. Finally, related to buying also the Country Level Domain versions of your main domain, actually buying them and redirecting 301 to the respective subfolders doesn't have really any SEO effect, but it may be an useful way to "reserve" those domains for a potential future use.