Engine wise, you need to look at 1.8T stuff and not S3. The 1.8T Quattro engine has a single intercooler with the inlet manifold on the same side as a normal 1.8T sport, the S3 has twin side mount intercoolers with the inlet rotated 180 degrees. Transmission wise, the 4wd system is pretty much the same, but the S3 runs a wider track with different front struts that allows for little camber adjustment, not too sure about the rear suspension differences though.
HTH