OK, Hook up your 2 batteries in parallel, add the inverter and see how that works out which will help to determine your future upgrades and as such is a resonable plan.
Read up on how to load balance wire 2 batteries in parallel. This is the best way especially for 2 new batteries. Might as well do it right the first time.
Water pumps use very little power because they don't run very long. My coffee pot uses 1025W or 8.5A AC. That's 95A DC for say 15 minutes is about 24Ah, a significant draw. And with your small batteries the voltage may drop enough that the inverter shuts down. Especially in the morning after a nights battery usage. I have no experience with anything smaller than 2 GCs but others are better informed.