I just bought my Audi 90 Quattro 20v and I want to tune hardcore. So I was wondering where I could find a 20v Turbo engine cause mine has about 250 thousand miles on it which is just too old to tune. I am also open to any suggestions on where to start the tuning process. My goal is to get 400 to 600 hp in my car eventually...

I tuned my 20V Turbo and it has 260k on it...

What have you done?

Swapping a turbo 20V for your engine is not simple. Significant metal must be cut and fabricated to make room for the turbo and intercooler. You need the RS4 bumper to feed air to the intercooler. Price one sometime :-0 Then there is wiring and the ECU. 2Bennett Audimotive does one such swap every month, they have it down to a science and even still there is 100 hours of labor involved. Even the steering linkage must be changed to make room for the turbo! Figure $12,000-$15,000 for the swap.
2Bennett has a better idea. For no more than $4500 out of pocket they can put a 4.2 liter V-8 in the front. It is 75 pounds lighter than any of the five bangers. It makes 300 hp. Sounds like fun to me.