Obviously goods replaced overseas would in most instances cost much more than equivalent goods purchased in South Africa and you should always insure for the replacement value. You are not doing anything illegal by declaring two separate values. At present SA Customs appear to have no problem with 1 x 20' container with a customs value of just under R50 000. It is not necessary for you to have an actual delivery address before shipping your goods. Clearly, you should know which town or city you are moving to.
Most people arrange for an estimate or quotation if they are moving with the next six months or so.
This enables you to budget your expenses. Please remember that international removal costs do not come off your overseas settling in allowance. The actual move is normally booked one to three months in advance. It is of course possible to book moves with a much shorter lead in time, but then it helps if you can be a bit flexible with your packing dates. You can store your goods in a container in South Africa for one month if you are shipping in an exclusive container.