Yea they'll fit. Bit of a plain looking wheel IMO but look OK if your on a tight budget.
On the
Ibiza 8" wide wheels are the widest I'd go on the front arches, and probably 9" wide on the rear. And then the size I'd go for 16's or 17's because 18's look too big and 15's or smaller look too small. Also depends how low you are. All my opinion of course!
ET (offset) is the distance between the centre of the wheel and the hub mounting surface. So the
lower the ET number, the more the wheels will sit flush/poke out of the arches, and the higher the ET number the more the wheels will tuck inside the arches. But it also depends on the width of the wheel.
And when you add spacers that reduces the ET (eg 20mm spacer combined with a ET 35 wheel will make the ET 15).
I'd say ET20 to 40 is alright since standard FR/
Cupra wheels are somewhere around 35. Maybe even
lower ET, just depends whether you want the wheels to be flush with the arches or poke out a bit or tuck in under the arch.
That's my understanding of it all, but I could be wrong with some of that so maybe someone else with more knowledge than me could correct me if I'm wrong.