In my view Winter Tyre are a no brainier decision, especially if you are buying a second set of wheels. And 18" are the size to go for as the tyres available are far more comprehensive.
I may be biased as the Winter tyres I put on my 530d BMW were a transformation, even living in warmer South West England. They performed so well on cold wet roads as well as ice etc. just like driving on summer tyres one warm day. I do a lot of mile though in Europe and for many years my colleagues have thought I was bonkers driving with summer tyres. The law change last year making Winter Tyres compulsory in Germany forced me to buy them and I am so glad I did.
The Evoque has loads of electronics that make it better than any other vehicle in its class, and beyond, at getting traction and keeping you in control. But the tyre grip with the road is pretty fundamental to being in control. The terrain response system can only help so far if the tyre is not gripping with the road. Take a look at YouTube and search for Winter Tyre Reviews. There are a number of comparative tests that show how superior winter tyres are over summer and all season.
If you are looking for a source of winter tyre look at
http://www.reifen.com and get them imported from Germany, loads less expensive that UK suppliers. I did for the BMW and very happy with the fast delivery and service.
I have a set of 18" Evoque Wheels, delivery mileage with Dunlop Wintersports fitted thanks to my local Stratstone's. They were brilliant on my BMW and I expect no less on the Evoque. Check out the tyre review site for comparisons of winter tyres, yes from Germany but they are so much ahead of us in UK on this, I trust the reviews.
www. (and then) tyrereview.co.uk/Article/2011-Auto-Bild-4x4-Winter-Tyres-Test.htm. (Had to do it this way as the full web site address kept changing on save any other way)
Just waiting for the temperature to drop another couple of degrees and the 20" with Michelins will be off, cleaned and stored ready for next year.
Yes you must inform your insurers and if you get push back from the person on the phone ask them to consult with the underwriters. I have not had a premium but have asked for a note to be added to my file for future reference. They were more concerned if the wheels were bigger than original fitment.