Post a link to the wheels you're after. Basically, offset is labelled as 'ET' then a number. For example ET40. The lower the number the further the wheel will stick out towards the outside of the car. The wheel width has nothing to do with this. I'd imagine a Corsa will use something around ET40 as standard meaning anything towards ET20 will need stretched tyres. It can be done but some cars require a lot of arch work. Whether it be rolled arches or pulled arches or both.
Going up a wheel size isn't really a problem as long as you adjust the tyre size to keep the same rolling radius. You also need to take into account how low the car is and whether or not you need alter the camber you run.
To run ET17 on the rear of my E36 instead of the standard ET40 I had to roll the arches, run a slight stretch on the tyres and use -3 camber. The fronts weren't as much effort.