Don't go too much lower if you want to maintain majority of factory comfort.
I have been looking into the exact same thing over the weekend and read a lot of conflicting information as is always the case with things that can very much come down to personal opinion.
H&R, Eibach and VWR (racingline) seem to be the main 3 if going springs only.. wouldn't go anything cheap as your asking for trouble.
All 3 companies specify they manufacture a spring to work perfectly with stock dampers.. how many miles are on your dampers would probably determine if you require changing any other hardware.
If dampers and top mounts are good then just springs is fine, if you want to spend significantly more then a damper/spring set like bilstein b12 or VWR suspension kit could be options.. with new top mounts. Just the suspension kit is going to be £500+ mind.
One thing i've found strange during this research is eibach pro kit only seems to come with front springs, there is no way i'd only change 2 springs, just couldn't do it.. would feel like changing brake disc/pad on only one side of the car.
My local Audi independent recommended VWR Springs with H&R/Eibach ARBs but now after all the reading i've done i'm still totally undecided and no further towards pulling the trigger! One thing I have ordered is a superpro ALK, fitted one to my mrs' golf mk5 gti and the impact is brilliant, hopefully its as effective on the S3!