The best thing that I can suggest is that you get a DIY Kit and ask the guy at the store for a speal discount if you remove 2 of the cameras and cable. This will bring the price down from AU$2000 to AU$1350
This will film all the time. Ratio works out to be 1Gb per camera per day. However this can vary depending on what setting you got it on.
And the footage works on a First In First Out (FIFO) basis which means that as soon as the hard drive is full, it will start recording over the oldest data first.
Please note that I am talking australian prices.