I'd say up to £50.
They need to connect via 3.5mm or optical.
Their main use will be for video editing purposes, so obviously it needs to produce not professional, but good audio. I'm not asking for audiophile level or anything close, just not tinny, poor midrange pap.
No size restrictions, I was just thinking that NOT having a subwoofer might actually be easier, but whatever.
Personal use. I have Z-5400's for pumping out music at stupid levels. Like I said, this is purely for video editing.
Cheers.