Hi Orthopod,
You need to consider couple of things, before deciding on UPS.
1. Are you using or do you plan to use inverter?
2. Are you planning to use only the projector on the UPS?
If you plan to use projector only on UPS, then make sure your UPS can take near about twice that load. I'd presume atleast 1 KV should be fine, most projectors output is near about 250-300W max.
Try to go for online UPS, they have better charging and offer better backup mechanism than offline ones. It will also give you a display about load information etc.
I've taken a 1 KV Champion UPS.
Hope this helps.
Neo.