For that base car rack system that you need to install in order to attach the cargo box to the roof, I recommend going with the Thule 400XT Aero Complete Roof Rack Base System. The Thule 400XT Aero car rack system includes four 400XT Aero feet, four FitKit 2047 Clips (specific to your 2007 Volkswagen GTI) and two square load bars. The 50" length square load bars are the recommended size for the Volswagen GTI cartop.
http://www.orsracksdirect.com/thule-400-aero-complete-roof-racks.html
Then for the cargo box I suggest going with either the Thule 604 Ascent 1600 cargo box or the Thule 602 Ascent 1100 cargo box. Both connect directly to the Thule 400XT Aero base rack bars. The Thule 604 Ascent box offers more cargo volume and is longer if you plan to carry long items such as skis, snowboards etc. The Thule 602 Ascent box model is ideal if you are planning to just carry luggage gear etc. Both are the ideal length to match your VW GTI however, with little to no portion of the boxes extending over your roof top.
http://www.orsracksdirect.com/thule-604-ascent-1600-roof-box.html
http://www.orsracksdirect.com/thule-602-ascent-1100-roof-box.html