The lower bumper panel is actually GTI specific with towbar cut out, (their is a non GTI lower panel but also a GTI one, you need to buy the lower bumper panel and the detachable cover they come as seperate components. The lower bumper panel is totally different from the regular golf lower panel, it is the same as the normal GTI with a cutout and cover.
I bought a westphalia towbar off ebay about £100 some odd IIRC (new) fitted it to my MkV 170 diesel, when I sold this, I then swapped it, with a new GTI specific lower panel to my mk V GTI. you also need a switch box to be able to connect the electrics to a canbus system I think this was about £15 but can't really remember, not really a difficult job if you are handy with spanners and the OE spare wheel interior even has a cut out for the detachable hook when you remove it, I think the total cost was about £200 with new lower panel, cover, electric hook up kit and towbar.
BTW I love it, with a tuned golf 275HP and 315lbft it really kicks butt with a large trailer with two motorcycle trials bikes in the back, can still out accelerate most things and it stops with the porsche front calipers
(just need to strap the bikes in real firmly
)