I have tested similar arrangement with 350W solar panel and 120W 24V centrifugal pump via DC-DC buck converter set at 24V and no current limit. 
Idea was to have solar panel produce voltage above 24V and directly power the pump.

It does not work well. 

Pump was in a 12m deep shaft with a float switch set to go ON if the water level rises above it. The bilge well had sufficient volume to allow for overnight standstill of pump in worst case scenario so the pump as the sun comes up would pump the water during the day, and stop if level is lower than the switch. 

First problem was obviously power availability. I found that this arrangement worked only during peak sun input. 
Secondly, as this was centrifugal pump if it was started without enughuah power to overcome the head of the water column it would just be running and heating up the water inside the housing, as there was no flow. This caused premature seal failure and water ingress problems.

Much better was Solar > Charger > Battery > Switch > Pump!
This system even tough more expensive for the price of the charger and battery is much more reliable, the pump can start and keep the well dry even during the night. No rain but cloudy days would charge the battery and keep it charged as needed.
I found that even 18Ah battery was sufficient. System is in operation since April 2014 still using the same equipment.