Choosing a large, reliable input capacitor for switching regulator
For a buck converter like this, it is very important to have a low ESR ceramic capacitor directly at the input of the converter. Otherwise you will have EMI problems. 

So it is a common practice to put a small (10nF) ceramic cap directly at the input. For the bulk capacitance, you can then use the proposed 680uF (low cost) aluminium capacitor. (I would not recommend a tantalum capacitor for this).