Firstly get some prepaid SIM cards and check the data signal in the area you are most likely to use it. You may find some networks have stronger signal.
There are data only plans. Also compare the different networks and see what packages they offer along with the type of signal you are getting. Many offer month-to-month contracts, so the worst is you are locked into the first month to test and second month to give notice.
Don't forget the 3rd party data suppliers that piggyback on MTN, i.e. Axxess and Afrihost.
From what I can see, Telkom is offering one of the best data packages available, if you are in their coverage area. Vodacom seems to shy away from month-to-month contracts. So you stuck with 24 months or prepaid. MTN allows 3rd party piggybacking on prepaid. The others I don't know.
But the most important thing is coverage. This should be your starting point.