With the Safari Zone rares, you can use repels to help A LOT.
Most of the rares have levels that are in the midpoint of all the pokemon in the area, so by using repels you effectively double your chances of finding them.
Area 1: Repel with Lv25 pokemon in lead for Kangashkan
Area 2: Repel with Lv26 Pokemon in lead for Chansey
Area 3: Repel, Lv25 for Tauros
Area 1: Repel with Lv28 for Scyther/Pinsir
Area C: Repel with Lv23 for Scyther/Pinsir (doesn't improve chances very much)
It's probably easier to buy Scyther/Pinsir however.