We are HUGE fans of the 996/997 Metzger twin-turbo motors.
As you may know these use the same case type, crank and rods as 993 engines and add water cooled 4-Valve double overhead camshaft heads and water cooled cylinder jackets. They also have the advantage of being true dry-sump engines with the simplicity of an engine mounted oil tank. Oil cooing is done via a water/oil heat exchanger so there is no need for a separate oil cooler.
I bought a 2001 996tt in early 2014 and as of this writing now own three, plus four engines on the shelf. We modified the first car (Neo) to about 650 HP on a little more than a bar of boost by swapping out turbos, headers, fuel injectors, inter-coolers and adding some tuning. While we had the engine out we took care of the coolant fitting issue which is common on these and GT3 engines. Since then we have started using ProEFI tuning.