12. volts would be ok. . . But, if you are going to be running mostly AC Line-level devices and equipment, you would be far better off in the long run to start at 48V from the get-go instead of having to upgrade later.
12. V systems require very heavy wiring on the 12V side and copper is not cheap these days. 48V systems don't require such heavy wire, so the wiring will be cheaper. The 48V equipment usually has better features and in some cases may even be less expensive than the 12V equipment, and in many cases is more efficient.

As far as that UPS goes, I would just use it as it is for backup power, which is what it is designed to do. If the power fails, it will carry small, required loads for a time. I wouldn't permanently wire it into your house because that opens up a complete new can of worms. Using it as it is is already approved. Connecting solar panels to the battery to charge them as an alternative method might still be allowed, provided that thing is not permanently connected into your house.

I have used several different UPSs around my place for backup power. Example, I have a 7KW Ferrups unit that runs on 48Volts and puts out 120/240VAC. It lives out in an outbuilding and I have a long extension cord that I run into the house as needed to power some things I need if the power fails.

No code violations that way. . .

Doug