any deep cycle battery should work and will need perodic replacement
They DONT last forever! maybe 3 or 4 years

Your better off with ONE LARGE BATTERY because in parallel 2 battery
that look the same may not be identical and self discharge one another.
Different ages, and minor production variations can cause that.

just curious is there any way to drain your sump by gravity. lots of
people have sumps with pumps that could drain to say a low spot away
from the house but people never thought of it or didnt want to do the
digging