S4 rears can work out stupid costs really as sellers really cash in on the fact that they have s4 rears......
18z's are a straight bolt on , you'll need new brake lines such as HEL , and 345mm disks either S4 B8's or Merc ML 's , 4 bolts to hold the calipers on and depending on what disc you choose 1mm may need skimming of the centre bore hole, pads aswell.
There are some sellers out there doing kits for 18z fitting to B7's
The only real possible downside with going the 18z route is caliper clearance and your rims , 18" is the min dia , you have 19" so that's fine the offset is crucial or they wont clear the caliper.
rim style and offset will be the main point, S4 front are not so problematice as the caliper body is smaller to start with, either way spacers can help if needbe.