Turbo series?
In 2002 there was a change of turbo from August production onwards.
So, afaik, there is one turbo from 1998 up to 7/2002, and then another from 8/2002 up to 2007.
I believe that follows the change from 4 sp to 5 sp auto as well. And change from vacuum shut-off valve to electric servo valve on the intake manifold. (among other things)