DC-DC Step-Down (buck) converter 5V 10A 50watt, not understand how to size the supply
Is that going to be sufficient to drive the step-down converter when it's under max load itself?


According to your DC step-down converter the specified efficiency is 84% at 9-10 amps output.

So if its output is 50W and at a 84% efficiency

$$\eta = Efficiency$$

$$\eta = \frac{Power\ Output}{Power\ Input}$$

$$Power\ input = \frac{Power\ output}{\eta}$$

$$~59,52 = \frac{50}{0.84}$$

So your requirement for the power supply, with the converter operating at the full load is ~60W. So it probably satisfies the needs with 100W continuous power. At 18V input it should be able to supply a current of 60 / 18 = 3.33A. Well within your supply capabilities.

Be in mind that's simple calcs based on resistive loads and does not take into account a bunch of factors that can change power requirements a lot.

Efficiency formulas from http://formulas.tutorvista.com/physics/efficiency-formula.html