Just noticed you were from Ireland.
Recent post for a Grand Punto Abarth on 'winter wheels' and there's some useful 'bits' in this thread...
https://www.fiatforum.com/500/287701-good-price.html
Key thing is that you can fit a 'normal' Fiat wheel to a Grand Punto with the use of spigots (Halfords charge €80 for a full set - no partial sets) and wobbly bolts - but the cost is too much. The Grand Punto has the 'Vauxhall' gene with the smaller bore size (the hole in the middle) which is great for getting a cheap set of alloys.
Also since your model is the Sporting i.e. is not a T-jet you have the smaller discs 257 which means that it should be able to take a 15 inch. Double check that if you get a steelie that it clears the calipers on the front - some had trouble with getting a 16inch steelie to clear on a T-jet (ack. to Andy) since they have the 280 discs.
Your handbook should have the recommended wheels including the spacesaver. Also the difference in weight between a normal 'steelie ' and a spacesaver is about 7-8lbs - only 'real' benefit is the saving in space. You'll probably find that as you increase the size of the wheel e.g. to a 16 inch with a wider width of 6.5J / 7J that the ET will 'go down' to an ET39.