Gail, the major difference between a pool and a pond is the fact that you require bottom drains and your floor should slope towards the drains and the sides must also slope inwards.
You also require bio filtration and your sand filter should be replaced with a koi (gravel) filter (Optional) and you need venturis on the outlets for more oxygen on the opposite side of the bottom drains.
You will also need a UV light in one of your outlet plumbing lines to sort out the algae.
You can use your existing pump but remember that your pump will now have to run 24/7 and should turn the water around every two hours and should deliver at least 20 000 liters per hour in your case after the conversion. The average pool pump can do that.
A fairly cheap option would be to do the following:
[You must be registered and logged in to see this image.]I mean ledge for dogs and not latch
You can put a vortex in your pre filter chamber for better results. (Optional)
The settlement chamber and filters must have an outlet to waste to enable you to flush them from time to time.
Your pump should suck water from the last chamber and from your existing skimmer box with a valve to control the amount of water from each source.
You will have to replaster with a plaster mix and water proof sealent like Coprox or Sika to sort out your leaks and make the corners at the bottom round.
Gail requested a quote. Is there anybody in the Cape Town area who can quote on the conversion? If so, please send me a mail.