Zach, the standard cooler master fan that comes on the Hyper 212 Evo is pretty decent on its own and configuring it as push pull only gives negligible cooling performance increases. I have mine set up in push pull and the results were less than expected, but only you can decide if the purchase of an extra fan is worth the extra couple degrees difference. If I had to install another one I would not install two fans. It also depends if your motherboard has two CPU headers, if it does not then you need to use a fan splitter off of the single CPU header which would be an additional cost (low but still an additional one).
If it were my system I would replace everything with Corsair SP120s as I said earlier, but then again that is my system and I have the budget to do such things. If you want to spend the least amount while still getting good performance then I would just add a couple of SP120s as case fans and install a Hyper 212 Evo with the stock fan (or an SP120 if you want something more quiet and that will match the rest of your build); however, once the case is known then I can give a better recommendation.