You need fans for both functions. The MUST be in balance - in fact, total air flow will always establish its own balance. You cannot exhaust air from the case without adding air to the case from outside - you can't manufacture air out of nothing! So what goes out WILL come in. Your task in setting it up is to help that process so that supply is nearly equal to exhaust. Then comes the great debate over whether the net balance should have slightly more intake (so the case interior is slightly pressurized and air leakage out of holes goes OUT of the box), or should intake be slightly less than exhaust so air leakage is INTO the box? I'll refrain from offering my opinion here, in the interests of NOT hijacking your thread into another debate.