Like saddad mentioned I too am curious as to where you would put the guttering ???
If you are contemplating putting it at kerb/base level how will you contend with the second requirement in the advertisement?
For the system to work ensure the water butt is a minimum of 12" (30cm) above the plants to be watered.
The fellow on the next plot to me has installed a solar panel to his hut and to this he has wired in a pump which in turn feeds a drip system. At this stage I am not sure how he is going to fill up his water barrel/tank ???
My guess is he is going to 'tee' it off the general site water supply.
I am looking forward to the methods he is going to adopt this summer.