There are a few companies in South Africa that give King Air ratings. I know that NAC in Johannesburg and Federal Air in Durban do these ratings. Not too sure of costs but will probably be cheeper that going to Flight Safety for example, but then again, don't expect the same quality.
As to your question of a rating helping you get a job, all I can say is that you will have to join the end of the queue when it comes to King Air jobs in Africa. Do yourself a favour and ask around the King Air operators to have a better idea about job availability before you go and spend a packet on a rating that might not get you employed.
My advice - try and get the job first and then commit to the rating.
Good luck - hope you come right.