I have no experience with that brand, but $1200 for an inverter, a small battery charger, and some transfer relays and logic, seems a little steep.
I'm a DIYer and would buy a 2000 Watt steady/4000 Watt peak Coleman inverter, a small smart-charger to maintain charge on the batteries, and a couple of relays to activate the inverter and transfer the pump system from line power to the inverter. Those components would cost about $300.
I would buy some deep cycle batteries locally; about $200 at BJs or Sam's Club.
While I'm doing it, I will spend about $150 to put together a duplex control system so two pumps would be operated in an alternting/backup arrangement.
The battery system is for the "Nobody is home when the power goes off" condition. I will have a small generator, because we have had outages as long as 20 hours which is more than the battery can handle. The generator also lets me run the furnace, refregerator, lights, and a little cooking capability.
Because we use fuel oil for heating where I live, the generator will have a diesel engine, so I will always have plenty of fuel.