When choosing an ISP, fast speed and reasonable pricing should be your primary concern, but it will also depend on which ISP providers are available in your area. To establish this, you can check https://www.findmyfibre.co.za/ to see which ISP providers service your area.
It is preferably to choose a well-known ISP provider such as Liquid Telecoms or Afrihost that can provide you with excellent customer support and after-hours service. Also, check what additional installation or equipment costs there may be as well as any data capping that may apply to your selected package.