i dont think spacers have any significant effect on handling - they make the car look better. the standard 7.5" wheels with et54 offset.
adding a spacer effectively reduces offset by making the wheels stick out more.
going to 8" wheels with the same 54 offset would be equivalent to adding spacers of 6.3mm (1/2 of the extra 1/2" width is outside, 1/2 inside) - most people go to 8" wheels with offset of 45 or so, which is equivalent to adding 15mm spacer and just about fits on a lowered car without catching the wings.
so, you could add between 10-15mm - you need hubcentric spacers (they hold themselves centrally whilst you put on the wheels) with longer bolts and longer locking bolts. lots of people seem to like the h&r ones but theres plenty of choice.
alternatively buy some 18x8 wheels with an offset of 45 or slightly more if you can find them. some rs4 and rs6 reps are around with offsets of 45, 46 and 48
dont buy any wheels with a lower offset than 42 - even those may catch the arches and you would need to get the wheels machined to fit or the arches rolled - neither probably desirable.