If you have a DC back up pump I am not sure I would bother with the inverter. Or consider a like 120v pump to run through the inverter. Venting should be considered with a wet cell. If the area is fairly enclosed (hydrogen rises) I would get AGM batteries. Probably Lifeline and a 5 to 20 amp multistage charger that will safely float the battery. Two batteries (group 27?) is a good start but give it a test in routine conditions to verify run time and decide your comfort level.
I have looked at these systems for a driveway pump back-up. (3/4 hp, 12amps, 74 gal/min) Even with power off I would have at least 10 hours before damage so I just keep a generator handy. Heavy rain & power outage is very rare although we have had each just not together. I do keep a separate pump and long discharge hose for motor failure.
FWIW those are my random thoughts.
