Cost to change will still be 2-3k for a semi decent example. 8k S3 will be scraping the barrel. Nicer examples start from £12k.
This is what I found. There is a pretty nice S3 close to me, although it has just broken the 100k mark, it's a one owner, FASH, well looked after example. It's £10k. If you want lower miles than 100k, then the price goes up (obviously).
There is also a 60k-ish R32 (2008) local that is also around £10k. But it doesn't have leather or nav, and is 5 door (I prefer 3 door on the Golf, and 5 door on the S3). R32's and S3's seem to be roughly equivalent values. ED30's appear a couple k's cheaper, and the GTI's the same difference again.
To be honest, at this end of the market, mileage is not so much a concern to me. The ED30 I should be picking up this week has 97k on it. But it was easily the cleanest mk5 I have seen in my searches.