I'm hoping to find a DC to 230 VAC inverter IC that "has everything inside" except for a few external components (like FETs or IGBTs). May use coils but rather not a transformer.
I would just think about buying a UPS based on your post. That or a 12 VDC to 240 VAC basic inverter, the kind that plug into an automotive system. Something like this would work for you. THey run about $45 USD.