As almost always in this world the answer depends on your needs.
First of all you need to be aware that if your writer is non-native, native readers will likely notice it. I have met really good Spanish speakers that weren’t native, but you always notice something in their expressions. Is it bad? It is not necessarily bad, but you need to look at some points to decide if this situation is ok for your texts.
When should I hire a non-native Spanish speaker to write a text in Spanish?
Easy and short answer: When this writer will add more value with his ideas and knowledge than the value that will be missed because of the fact the text is not 100% natural.
If you need a text about a specific topic in Spanish, and you know a non-native speaker that is the best in this topic, I am sure your readers will be ok learning from this expert. No matter if there are some little mistakes.
When is a bad idea to hire a non-native Spanish speaker to write a text in Spanish?
When you decide to do it, because it is cheaper.
Actually, this bad decision would be the same if you choose a bad writer that is a native speaker. If you give a cheap text to your readers you are saying them that all your products/services will be created in a low cost way, which is not usually a good marketing technique.
So first rule is to avoid bad writers, no problem where they are from.
Now, let’s think you have a good writer, but he/she is not native. If this writer isn’t the expert that I have talked about before, my advice is to choose a native one, because for Spanish speakers read a text written by a native is always friendlier than read a text that looks kind of not Spanish.
Show you reader you are worry about them being comfortable and make it easy for them.
What Spanish location is the best to produce a text in Spanish?
It depends on the target of your texts. If your texts will be mostly read by people from a specific location, the best option is choose a good writer from this location.
Argentina, Mexico, Cuba, Spain, each country have its own expressions and, of course, very good writers, so take it easy and choose one that is accurate to your target.
But always keep in mind quality. I, as Spaniard, would always prefer a well written text by someone from Mexico that a bad one from my neighborhood. A good Spanish, no matter where it was produced is understood by every Spanish speaker in the world. Bad text is hard to understand even when the culture and history is common with the readers.
If your target is international, I would recommend hiring someone from Spain. The language is as good or as bad as in any other place, but it is taken as more neutral by many readers. Maybe this will change with time, I am not saying Spanish from Spain is better or purest, just that historically we were looking at it in that manner.