Needing small system for dialysis machine

Hello, I’m new to EG4 but in my search for a small battery system, I started seeing them more and more.  What I’m looking to do is this: have a basic home battery back up system, no solar at this time (would like to add small emergency array in the future), that will allow me to run basic lights, router, fridge and a dialysis machine for my teenage son.  This machine uses approx. 550 watts intermittently when running and needs to last for 3.5-4 hours.  It bounces back and forth between 550 and 125 watts every minute or so.  I also desire the switchover to be as Instantaneous as I can get it as any blips in the power give the machine fits, it sets off alarms, etc.   I heat with wood in the winter so no furnace backup needed and I have a portable AC unit that can work in a pinch for AC if needed, but it’s not a requirement.  I have a good inverter gas generator so I’d like the ability to charge the battery/run the house if need be and that option isn’t available with some other, more expensive batteries.  My game plan is to install the equipment myself BUT have an electrician handle the wire connections and verify my work.  Hopefully I can find one that will do so.  

What I THINK I need from EG4 is: grid boss, flex boss 18 w/ conduit box, and one 280 AH battery.  I have some room on the wall of my insulated garage right next to my breaker box to install this.  Am I on the right track with what I need for this scenario?  Thank you in advance for any assistance you are able to provide.