Those are some nice wheels mate! I'm more of a white alloys type of person but everyone has their own preferences!
When I first got my Punto MK1 it had simple stock steelies which were 13"s, then I've bought a set of 14"s off a Bravo and fitted them that day, so no extra bits n' bobs
In my opinion with 15"s you'll have to widen the wheel arch and will only manage to lower your car by a little bit (remember I'm talking MK1s here!). Unless you get the tyres that go over the rim (so fatter), but the are legal issues there, only the stocked, 'normal' tyres will pass a MOT.
But hey! No pain, no gain! Firstly I would try any 15"s (even if you could borrow them from a mate or something) and then if all is good, then get the ones you want
Hope this helps mate