General UPS questions

Goal:
Planning to get some sort of cheap and simple ups that would be able to keep my pc+modem+1-3monitors 100% reliably uninterrupted by a potential power blackout for at least a minute (just to save work / inform people that i'll be going offline) and at the same time offer some basic protection against power "instabilities" and such that occur during bad weather here and could possibly cause a crash or damage. (During a storm sometimes lightbulbs dim out for a second but pc still somehow keeps running, gets me scared. Also the badweather-caused full blackouts here usually involve the power going on off on off few times rapidly within a second or two before staying completely off or stay on again)

Main question:
I got a wattmeter so i will try to analyze things thoroughly before buying a specific ups, but for now lets say the sum of all the things that i need to be kept running at the point of blackout is 450W (pc under full load, monitors, modem). I went through the ups faq here and read wiki etc and if i understand it correctly, i need a line-interactive ups. So would a 800VA / 500W ups be enough for the job or should i go higher or much higher to be absolutely sure that it handles the switch to battery power 100% reliably? In other words, is the risk of things still turning off during a blackout higher if im very close to the number of watts the ups is rated for compared to having say a 1000watt ups and only needing 450W to keep my devices running?

Other questions:
- If i need to connect more devices than the number of outlets on the ups, is it ok to just put splitters in there and connect my stuff? Assuming i stay within the watt range that the ups is rated for and the devices aren't laser printers etc.

- Iam planning to add a simple instant pc power killswitch between the ups and pc but the only suitable device i found already has some simple passive surge protection in it, could this really cause some trouble during regular run (as the faq says) or at the point of potential blackout? (i dont have it yet but i guess ill try to remove the surge protection from it)


Thanks for any kind of shared info