Don't overthink this.
Buy the biggest battery that will fit. For that size specs are very close brand to brand.
Get a deep cycle type, they are built different.
Important - if a 2 battery setup, buy 2 -6v batteries. Why, slightly more power but more important, batteries in series are safer, do not have to be matched. Even new batteries built the same day become mismatched over time.
Get at maintenance free battery. Watering batteries sucks and if one goes dry it never comes back.
Don't be cheap about this, good batteries cost a more.
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