Peter's got it right. Decide what Spanish-speaking countries you want to target and buy country-specific domains. We are Costa Rica based and work on a lot of English-Spanish sites. For G searches performed here in Costa Rica, co.cr domains tend to show up on top no matter where they're hosted or if they're nicely optimized or not. But perform the same search from Italy or the US, and you get different results, in which case, .ar and .mex tend to do better than the other smaller country domains like .co.cr or .gt.
Depending on what you are selling, you will need to adjust the language a little bit. Subjects like politics, economics, medicine, history, and the like will read nicely in any Spanish-speaking country. But things like the names of fruits, foods, car parts, and household items may be radically different. Same thing with adjectives. Yes, there are general words that can be understood in most countries for a specific item. So try to choose someone that knows the differences and can write accordingly.
Avoid having someone from Spain write for Latin America, unless they have lived on this side of the pond for some time. To illustrate how different Spanish from Spain can be from Latin American Spanish, I'll tell you a little story. We took our Costa Rica born, Spanish-speaking 8 year old son to Italy for a few weeks. We traveled through several Italian cities. Then we went to Spain. After 3 days in Spain, my son asked for ice cream - get this - in Italian. He had not realized we were in a different country. For him, Spanish from Spain was about as unintelligible as Italian, and of course Madrid was just another city of weird old buildings. Just the ice cream was not the same, definitely.