The higher offset will move the wheel more under you car. As MikeR said, your biggest concern will be suspension and brake clearance. This however can be corrected with a spacer; a 10 mm may be enough. (You'll need to get new, longer lug bolts though) As long as you maintain the overall height of the wheel/tire combo, the 20" size won't really matter. (Bigger wheel with a lower aspect ratio tire) Remember, less tire = more road feel. Personally, I don't mind. Some do.
PS: Find a new wheel/tire shop. This kind of situation is rather common.