u dont need to take the car anywhere to do the arches, i was in the same position as you, but i had 16" and was debating wheather or not to go for 17" in the end i did, its the best decision ive made, the car looks amazin im well pleased.
doing the arhes is a piece of piss, all you need to do is take front and rear arch liners out and remove ventilation boxes, take the rear bumper mounts off and grind away the little piece of metal that surrounds it, if your fitting a 205 tyre it will be fine, i can have 2 peeps in the back and it dosent touch at all.
if your not bothered about performance do it the car looks amazing.
hope this helps