I'm going to save you some money and just say you should stay away from doing anything major to any turbo VW. I hate VW's regardless because of styling, but they are always having problems @ above 300whp mark. My friend had a GTI and it was a piece of shit.
B5 S4's are great little cars if you want turbo / german. You could also turbo an e36 with relatively good reliability, but you better have $$$$$$.