I thought Beuran was the best of the bunch. Marchant will fetch. Parnell will fetch. Shezi will fetch. Pitch won't assist Abbott so he won't be threatening.
About 2 years ago, albeit in a T20 triangular, Zimbabwe comfortably beat the Proteas twice. That Proteas squad contained Amla as leader, Faf, Duminy, Marchant&Parnell. Squad was strengthened after Faf had disappointed leading that side in the List A series before that but it clearly didn't help -
http://www.espncricinfo.com/zimbabwe-tri-series-2012/engine/series/561548.html
In the List A Tri Series ZIM had left out Taylor, Masakadza, Sibanda, Price, Utseya plus Cremer(who like Tahir is rubbish at Tests, but gold dust in LOIs). While the Proteas won, they never convinced -
http://www.espncricinfo.com/ci/engine/series/565849.html
With all of this in mind, plus looking at the Proteas squad, I think they weakened the department(bowling) that disappointed before. Unless Amla&AB are top form, these 3 games might be closer than the Test!