Is it reasonable to convert a general purpose motor and pump for a sump pump?
Pumps where the pump mechanism (the impeller) is above the water line must be primed. So there is water in the impeller and no big airbubbles in the inlet pipe. If this is not the case then you can damage the pump.

You'll need a non-return valve at the inlet to prime it but if it leaks then you cannot keep the pump primed for any extended amount of time. 

You could create a prime-on-demand mechanism where when you need to pump first a few gallons of water is used to prime the pump after which the pump can activate.

The cheaper solution is to run a shaft from the motor to the impeller (like a pedestal pump) or submerge the entire motor/pump. Only after taking precautions for waterproofing the wiring and ensuring adequate cooling.