Brand is going to mostly depend on where you live and availability. As long as you stay away from junk shipped directly out of china you should be ok.
What you are looking for is a UPS that can deliver the full 1000 watts your power supply might pull. Likely it never get that high but if you were to exceed it the protection circuits in a UPS would turn off the power as if you unplugged it suddenly.
Be careful VA and watts are related but not the same read the fine print and look for how many watts it will provide.
My guess is you are going to be looking at a 1500va unit.
The key feature you want that will make it more expensive is you want sine wave...not simulate sine wave. Modern computer power supplies are very sensitive to poor power. If money is really tight it is important to remember that you are only running on this simulated sine wave when you are running on the battery. Depending on how often you lose the power it likely is only a small number of minutes a year.
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