well to be honest i don't anything really in modding mk4's...
i would say do the basics first...
exhaust
filter
intercooler
boost controller
decent blow off valve
then upgrade to a single turbo i think
fuel pressure regulator
aftermarket ecu
and well after that i wouldn't have a clue lol
try the mk4 guys on supraforums.com and see what ya can find there... i suggest you do a search first then post your question in case you repeat someone elses post... if you repeat posts the search police will be on you hehe
let me know how ya got on and what info they gave ya
good luck