Wheels are a funny old business as you need to consider stretch, poke, offset, the width of the wheel, camber and how much comfort you want to retain.
Your best bet would be to find out what your current ET is on your wheels as that will give you the starting point in terms of where next.
Once your done there, find out how big a wheel you can physically fit under your arches, you are legally allowed to have the rim poke a massive 3cm outside the wheel arch but the tread on the tyre must remain inside the arch.
Without doing any work to the arch, I reckon you could fit 8s on a polo with a bit of effort. 7.5 sounds good also. Have a look around to see what people have done on their polo's as I don't think many people have your shape on here