get the a4 1.8t, you cannot and will not be able to turbo the 6, there is a supercharger kit but it takes some tinkering and many people have had trouble/blown engines. get the 1.8t

big turbo kit
brakes
suspension
exhaust
intercooler
clutch

then you will have spent $10-15k and you are done! you will have a 350-400chp vehicle that still gets 30mpg on the highway!

 

or if your not big into modding, get the 3.0. i have a b5 2.8 and it hasnt cost any more than synth. oil changes and premium gas over the last 2 years. the maintenance is expensive, but not as often as rumored.