I am having a set of 18 inch et29 A5 alloys powder coated satin black at the moment which will then have Bridgestone LM005's 245/40/18's fitted, sourced from Oponeo as was no stock in Uk at time of order.
From what I have ascertained, the offset will nicely clear the s4 calipers, so should be fine for your s-line too.
Pick them up tomorrow though wont fit them for a couple more weeks...
A few sets on ebay from around £300
Part no is 8T0 601 025CC
17inch wheels will fit on the 2017 B9 A4. I got the genuine winter wheels from a B8. The offset is different but use 10mm spacers with them that I normally run with my summer wheels and that corrects the offset virtually to what it should be in the B9. 17inch wheels do not look great on an A4 but wanted to have the option of fitting chains on them when I go to the Alps. Apparently you cannot fit standard chains in 18 or 19inch wheels appart from some few very expensive ones. If you want to run chains then your offset needs to be correct to give the chains enough clearance.