Garret turbos are good for the 1.8t. Only because they'll fit, and they're cheaper than KKKs. Most turbo upgrades use garret for reasons that APR mentions on their site:
http://goapr.com/Audi/products/stage3_long.html#
but a lot of companies have products.. check the VW/AUDI OFFICIAL DIRECTORY for some of those like AWE tuning, MTM, ATP, REVO, APR, etc.


If it were me, id start small. Just to make sure there arent any major problems with the car since you bought it used. start with a chip.. then move into exhaust, then intake, then make a race n75 valve, and a diverter valve for safety, then you can get into a downpipe, maybe a manual boost controller and a adjustable FPR with some tweaking. then upgrade the turb. Do some research, thats the best part.
Check audiworld.com for lots of info on anything.