Obviously it's not an OPTIMAL situation, having only one fan, just make sure that the one fan is installed in the rear fan location as an exhaust fan and keep an eye on temps until you can at LEAST add a second fan in one of the front intake locations.
In a perfect world you'd probably want to end up with two front 120mm fans as intake, bringing in cool ambient air, and a rear 120mm fan as exhaust, taking out hot air, AND a top rear 120mm fan, also as exhaust, since the stack effect dictates that that is where most of the heat is going to accumulate at.
Even some cheap fans would be better than none, but you should be ok temporarily.
Might not be the worst idea to add a cheap intake fan temporarily if you plan to get better fans...