+1 with JFP
The 911TT and GT3 engines are based on the "old" Metzger designed air-cooled engines. As such they do not have the problematic IMS bearing or head cooking problems that were more prevalent in the earlier water cooled M96/86 type engines.
I do know that when the water cooled car first came out. Parts for engine and trans rebuilds were nonexistent. Now 10+ years later parts are available, the cars have been heavily tracked, weaknesses that have shown up have been addressed, DIY folks are rebuilding them etc.. There is also a pretty good independent support now.
Sound like the 901/911 history is being continued?
I have more experience with the manual trans the biggest pain is that one needs access to a robust press to disassemble one of these transmissions.
In the manual 6 speed box Porsche has also continued the "weak 2nd gear syncro" tradition. Although you have to beat it pretty hard to break it as often as the earlier 901/915 boxes. GBox among others has one answer for that, if you catch it early.
http://www.gboxweb.com/detent.html