Converting isn't necessarily a bad idea if you've already got a foreign CPL, but getting one just to convert it would be. Both the SA and EASA CPLs require 200 hours. So getting one then converting to the other will mean you end up using more hours in the long run. As mentioned above there are reductions in the course - but the CPL course is only 15 hours anyway (with an IR) so it's far cheaper and easier to just do that.
Where you will save a fortune is with the IR. Do it early and you can hour build IFR, killing two birds with one stone. With a foreign IR and 50 hours IFR PIC you are exempt all training for the EASA IR (You'll need some, obviously)