GAPS water or Pure Freedom.
IMHO, Purefreedom sell a fully setup system on a frame - no messing around trying to work out how to put it together. They also include 20" prefilters whereas the basic kit of GAPS has 10" prefilters. To compare apples with apples, Purefreedom also put resin in the DI Vessel, so when comparing prices you need to add the cost of resin to the GAPs unit as well as a DI Vessel. However, Pure Freedom do charge more for an HF5 membrane.
You can save around £120 if you buy the components separately, but trust me, its not worth the extra hassle of sourcing components.
The 4040 is a big beast, so make sure you have some place that can accommodate it. Especially consider how you are going to get it warm over winter.