The batteries life cycle is 4 years, less with heavy repeated use. I know everyone is "buy online" but stand the chance of getting a fake battery, thats bad. SO go direct from APC or go to some place like Batteries+ in town.
Looked up that model, from the look of it that is WAY old. I would get a new unit. Looks like a BR700G is the replacement model of your old one $150 from APC, $105 from Amazon.
If you have one, I would set down with a something like a Kill-a-watt meter and see what the equipment attached is actually pulling, with that number and how long you would want the power to be on you can size the right UPS. This is best as these run most efficient at certain levels. Your wattage shouldn't exceed 60% of the VA rating... 1000VA, 600W.
APC also has configuration tools as well where you can put in either load values or an equipment wizard.
https://www.apc.com/shop/us/en/tools/ups_selector/
Also the Powercute software is pretty good about shutting systems down gracefully when the power gets too low (You can adjust the threshold)