Depends on location of cameras, don't go for wireless cameras as batteries need replaced. Even a old pc with a video capture card stuck in a cupboard some place in the house/attic out of the way and left on 24/7.
I use that at work & have remote access to the cameras so it can be monitored from here or Glasgow, London, china or any place with internet connection all with remote recording plus the main pc.
Peace of mind is priceless imho.