I was loking aty CCTV camera's on ebay - seems the best for night vision are these new(ish) LED camera's (rahter than a normal looking lens they have a bunch of "LED's" on the front - supposed to be really good in the dark....

I guess you can hook em up to a hard-disk recorder for constant 24hr recording - thats how I had a system set up at work when I was in security - bumch of camera's hooked into a hard disk recorder - recorded 8 camera's (all perfect pic too, not still frames like time lapse) 24hrs a day and had a total of 14months per cycle I think You could go back to like a year ago and watch again wjhile it was still recording!

Cheapest tho is a tim lapse video (records 24hrs then rewimds and re-records - unless you take it out and stick another tape in of course)