firstly, the brackets sold for the s3 will only work with the 993twin turbo brakes - the calipers are very hard to get second hand because every 911 tweaker out there is putting them on their cars. new they are £310 each plus vat
the disks are about £118 each which represents very good value for money - if you can get 2nd hand calipers they tend to go for about £500 a pair reconditioned, so
buying the bits new but separately is slightly more expensive than the complete kit from AMD or others
993tt calipers cannot take disks bigger than the 322mm.
second hand route will only save you £200 or so IF you can find the calipers.
i tried to find someone to use the 996 turbo calipers with the bigger 330mm disks, but i could only find one in canada that charges £600 for the brackets and bells...also the disks are about 10lb heavier each than the 993 ones
you could have a custom set of brackets and bells but the companies need your car for a week or so to do this.
which leaves bigger brake kits:
brembo floating kit 328mm very expensive
tarox big disk kits -not sure
r32 /tt 3.2 brakes 332mm but single pot sliding caliper about £1000 with all the bits for a proper conversion
AP 332mm kit approx £1400
hi-spec do a 355mm kit very cheap but the disk have significant runout, dont centre properly and the number of appalling stories on the net says dont bother...
ECS are currently working on their stage 4 big disk kit for a3/tt - it isnt on their website but the version for the golf it. it will use their own 355mm drilled/grooved floating disk kits with porsche boxter monoblock front calipers. i spoke to their engineers and the whole setup is ligher than the porsche kit with equivalent stopping power - directional cooling veins, very quiet in use, replacement disks are about $120 whole kit will be a few hundred pounds cheaper than the porsche 993tt setup. maybe wait for this one.