18" wheels are possible as even 17" alloys were fitted to certain models:
https://www.ebay.co.uk/itm/304508888900?mkcid=16&mkevt=1&mkrid=711-127632-2357-0&ssspo=ilpS14oASGe&sssrc=2349624&ssuid=IE07kO4STZy&var=&widget_ver=artemis&media=COPY
https://www.ebay.co.uk/itm/133883388923?mkcid=16&mkevt=1&mkrid=711-127632-2357-0&ssspo=mkq438n3qi6&sssrc=2349624&ssuid=IE07kO4STZy&var=&widget_ver=artemis&media=COPY
https://www.ebay.co.uk/itm/325153828924?mkcid=16&mkevt=1&mkrid=711-127632-2357-0&ssspo=4IRGjOU0TEK&sssrc=2349624&ssuid=IE07kO4STZy&var=&widget_ver=artemis&media=COPY
https://www.ebay.co.uk/itm/195092215221?mkcid=16&mkevt=1&mkrid=711-127632-2357-0&ssspo=8Se7mkv-QkO&sssrc=2349624&ssuid=IE07kO4STZy&var=&widget_ver=artemis&media=COPY
I'd recommend a diagnostic check to see what fault codes flag up. It could be the fuel pump, MAF sensor (would run rough before cutting out though), cam sensor, lambda sensor.
Welcome to the forum you'll find the members on here are a friendly and helpful bunch 🙂
It'll have to go to a dealer or an retrofitter like autologics could do it. But expect to pay £600.
Just had all mine done. Along with the glow plug relay. Only 3 of the 6 were working on mine. Also don't get cheap ones. Get bosh or NGK. Bosh are what Audi would fit when car was new. And it's bosh I went with.