I don't think anyone here can give you a definitive answer, JeanSP, but merely educated guesses. But let's see....
What happens when you move to Spain? Are you CERTAIN you'll be moving to Spain? So you don't already have a visa. Do you have or will you be getting Spanish residency? Marrying a Spaniard? Studying? Retiring in Spain? You cannot simply "move to Spain" and then register. You have to have a visa inhand upon entering.
Visas are complicated and hard to get - no matter which kind you want. Of course, if you're "rich" there's little worry but you'd still have to go through the process. One Spanish visa offered allows those with something like 120,000 Euros deposited in a Spanish bank and can prove outside income. I'm not at all sure (I doubt) this gives you any residency rights to public health care, for example, but you could always get private health care like "Sanitas".
Regarding the income, is the money made at home (South Africa) being transferred to a Spanish bank to which you'll have access or will you simply receive money transfer or be using only ATM machines? That's an important question.
The two country's tax departments don't have communication between them so one wouldn't know what's happening at the other. So again, while I don't know for sure, I don't think you'd likely be taxed in Spain from money coming from outside of Spain.
By the way, your website states:
My name is "*" and I have been an expat in Spain for just over a year now, I am originally from sunny South Africa
So.... You're already IN Spain??
Saludos, MadridMan
