The VA rating should be about 1.5 - 2x the power consumption of your components
The VA rating has little to do with how much the UPS will actually keep your devices running one power is down - that depends on the capacity of the batteries the UPS is supplied with.
You are probably not printing 24/7, so your printer is probably idling at 1-2 watts, and when it's actually printing it's maybe consuming 10-15w. You can pause print jobs when you notice power fails, so the power consumption of a printer is not a factor.
You wouldn't take this in account if it was a laser printer, which can consume 800w+ when printing.
Yeah, monitor's gonna be 20-50w depending on brightness
Your computer's gonna be 50-100w when idle (in windows, typing documents, watching youtube or a movie) depending on the configuration, number of mechanical drives.
It will only consume more power if you're doing some rendering job that uses cpu at 100% (add 50w) or when you're gaming and video cars runs at full power (that's 100-300w extra depending on video card)
A 500-650VA UPS will be fine, but you'd be better off going with lower VA and more quality instead of just big numbers.
off-line < line interactive < on-line ... ideally go for line interactive or on-line ups, on-line are expensive though.
then it's simulated sine wave vs pure sine wave, pure sine wave is better but most modern hardware can tolerate simulated sine wave just fine so it's not a big deal anymore
look at the battery capacity... the cheapest will have something like 7Ah battery, that will keep a 100w pc running for around 10 minutes or so and then take 8-14h to fill. So depending on how often you have power failures, may want to go with something that has more capacity, or option to add external battery.