If your mk7.5 GTD has 18" factory fit Seville alloys and assuming these are the same spec as the mk7 GTD 18" Nogaros, your current wheel/tyres are 18 x 7.5J, PCD 5x112, ET49, Centre Bore 57.1. Tyres; 225 / 40 R18.
Spec of the alternative wheels you're considering; 19 x 8.5J, PCD 5x112, ET45, (Centre Bore?). Tyre size; assumed at 245 / 30 R19. If the centre bore of these wheels is larger than 57.1, you'll need spigot rings to ensure a tight fit on the hub and you don't get wheel wobble or vibrations. You may also need new wheel bolts, if the profile of the bolt holes in the wheels differ to your that in your existing wheels. If these are brand new wheels without tyres, depending on tyre choice, 19" tyres can be quite a bit more expensive than the tyres on your 18" wheels.
The difference in clearance between your existing and wheels and the 19" wheels is;
Between inner wheel face and suspension strut; 8.7 mm less
Front face; protrudes an additional 16.7 mm.
If you're still considering various different wheel options, you'll probably find an offset wheel calculator useful. The one at the link below currently holds the above wheel spec and tyre size details;
http://www.willtheyfit.com/index.php?width=225&aspect=40&diameter=18&wheelwidth=7.5&offset=49&width2=245&aspect2=30&wheel_size=19&wheel_width=8-5&offset2=45