firstly on the mpg issue the newer mk4 golf enignes i.e the 100 bhp,130bhp and 150bhp are PD(pump dusse engnies) these engines are more powerful and nippier but the are not as economical as the older non PD engines like the 90bhp you have.
If i was in your situation i would actually go for a well minded UK 130pd one,the 150 suffers from serious turbo lag and the engine isnt as reliable as the 130,a 130 will remap to 170 easily and reliably.
If you do go for a 150 one i would still go to the UK for 2 main reasons
1. there are countless golfs been advertised on carzone etc as being 150s that are not 150bhp golfs
2. most of the ones that are actually 150s are uk cars originally and VAG TDIs are noe of the msot clocked cars out there.
Also dont buy into any of this GT TDI sh!te,there is no such thing,get yuorself a nice highline well specced up example and you have yourself a mystical GT TDI.
Dont pay attention to the rear badges either the only true indication of power is the sticker in the log book or boot,150 bhp should be 110kw.
If you are hell bent on getting a 150 then i would look into getting a diesel anniversay golf very limited in numbers and alot nicer,it comes with bigger wheels,bigger brakes,nice understated bodykit,proper recaro seats (thats another thing diesel golfs on carzone with standard VW highlin seats being advertised as having recaro seats!) brushed alu trim,bespoke steering wheel etc.