About the Variantes...there are, if you'll pardon the pun, several different variants. The Hyper Chrome is the one you want I believe. They have the correct offset to go right on a GTV. Sorry to say, but I think you'll have a hard time finding these as MAK don't have them in stock on their web site.There are other ones which have a different offset like (I think) the Hyper Silver Mirror. These can probably be fitted, but would need faffing. I would confirm with the company though or somebody who knows more about this. The ones on my car were ordered special through rimstyle.co.uk.
In regards to 17", 18", I have 18" but have never driven my GTV on smaller tyres, so can't compare. I was fearing the ride before I got the car, but it's fine. Suspension is in top shape, which helps. Hope that helps.