I would say do a good rebuild on the bottom end, and make it really strong.
Lighten, balance the crank, as well as knife edgeing it. Forged rods, pistons, etc...
Port, polish, etc the heads.
Get a good Rising Rate FPR, as well as a fuel computer, and higher flow injectors. Here in the US, a lot of guys use the injector from a Mellenia S, wich is a factory Supercharged Mellenia.
Then put on your turbo.
In the Bulk Buy section, the guys selling the big brake kit also make a turbo kit, so check out their website for turbo stuff.