Good find on the UPS. I buy those sized units all the time when I see them at auction. I bought a pallet of them a couple years ago, about 20 for under $50 for the pallet. Every single one of the ones I have bought at auction have worked because they all have the same fault, bad batteries. Battery replacement cost is so high, and it has been about 2-3 years so most places just discard the unit.
Inside the UPS is a parts superstore. Mosfets , each unit of mine has about 25 of them. Relays probably about 10, omron brand, not cheap stuff. Caps about 20 , nice nichicon branded. Transformer - massive 1:1 transformer with a 20A rating, heatsinks about 4-5 nice heavy ones, torroid cores used in filtering 4-5, circuit breakers, fuses, misc components. Really great stuff for building power supplies. These units are pure sine wave so lots of uses.
For batteries you can use marine or solar batteries and these UPS are rated for the stated wattage rate continuous not like the low end UPS that will overheat if run for 30 minutes.
The batteries can sometimes be restored depending on how they failed. Often it is because all the water has evaporated out of the cells. Use a hydrometer to check the electrolyte , often just adding more water can bring the battery back to a useful state.
Before anyone says "these are sealed batteries, they don't need water" they are sealed, but still vent over time. Take a sharp tool and get under the top plastic cover and snap it off, under that is 6 cell covers, little rubber caps, pull those off and check the water level. They have to vent the batteries because in an overcharge situation the battery could burst if not for the safety vent. The safety vent isn't 100% air tight so over 2-3 years the electrolyte level drops. BTW, car batteries are the same system and many car batteries can be prolonged or restored with added water.