if you wanna lower yours, go with eibach or h&r spring kits. can get them for around £140.
cant see used s3 springs being that much cheaper, so youre not really saving. plus used vs new. also factor in the labour, if it turns out to be ****, youd have to get the whole lot changed out again.
engines weigh different amounts (id assume the diesel is heavier), s3s have quattro vs most other 8ps being fwd. thisll have an effect on the rebound rate and the ride height.
just doesnt seem worth guinea pigging it