Hi Jack
Colin is definately the guy to speak to as he lives in Pretoria(just outside) and has many years experience.
With the final sealing, it depends on what you have done with your plastering.
A concrete pond may still have a very high PH, irrespective of what sealer you are using. I would not rush anything. If your ph is fine by the end of the month, you can safely introduce the fish. If not, just buy a portapool (R 1000.00) and keep the fish in there until your pond is 100% fine. You will need a quarantine and hospital facility anyway. When your pond is complete, you purely pack it away untill further need.
There is no need to transport some of the water to your facility. Purely take some of the media, keep it in water, and transfer it to your new pond or portapool as the case may be. It is winter, and this aspect and maturity is not crucial. As long as you limit feeding and do regular water changes initially, and monitor your Ammonia and Nitrite levels.
As far as the borehole water are concerned, you have to test it. Put the parameters on the forum or compare it to the figures given on the forum under water conditions. Normally municipal water is better, as long as you take the chlorine out before using it. Colin will help you here. Some areas in Pretoria may have excellent borehole water, and a few hundred meters away it might be bad.