Ahh, yeah, I know the Costa Del Sol villa, if you have the money early enough (which would probably only be made through massive amounts of levelling up) you can buy it as early as the first crossing from Junon.
I just found the exact wording you used (refering to it as a "mansion" - I usually see it refered to as a "villa") a bit off-putting.

I do find myself wondering, there's a guy in the basement when you go looking for goodies (there's a couple of items, one in a chest) who can still be found there even after buying the place. I wonder who he is? The housekeeper? XD