The Free Dictionary website states and so does Wikipedia:
n pl erven [ˈɜːvən]
(Engineering / Civil Engineering) South African a plot of land, usually urban, marked off for building purposes
The term erf is used in legal documentation and technical English throughout South Africa and Namibia, maybe even further afield in the region.
Is it used anywhere else, except maybe the Dutch/Flemish regions in Europe, where I assume the term originated?
The client purchased Erf 132, Windhoek for a large sum of money. He intends to build a house on this erf.
The capitalisation of "Erf 132" is commonplace, since it is a definition of a location. This is similar to capitalisation of towns or cities. I find it slightly awkward, but that is how it is done.