On the invertor, I'd get the 300 or 450, there's not that much difference in cost. You might use it for additional items as time goes on: hearing aids, AA or AAA battery charger, 18VDC drill battery charger, some WiFi boosters run off of 110VAC, PC printer, and I'm sure there's more. Yes, using two 6V batteries together is a good idea.
We have a couple of WiFi extenders. I like the Crane unit mounted on the top of my ladder and the cable run inside.
Crane Antenna ClickyThey have a couple of different models.
Bill
Nodwell RN110 out moose hunting. 4-53 Detroit, Clark 5 spd, 40" wide tracks, 10:00x20 tires, 16,000# capacity, 22,000# weight. You know the mud is getting deep when it's coming in the doors.