Once cars get to 5 years old, not all cars are equal for condition, service and history. It doesn't always pay to buy the cheapest. I would be very cautious looking at a £12-13k GTi personally.
Let's see how the mk8 turns out, if it has any hint of hybrid technology I will not be looking to own one until I have to. The mk7-7.5 may be seen as peak Golf???