indeed 18k for that carrera engine sounds like a lot. 2 years ago a friend had a 3.6L rebuilt for his 996 by a reputable company here in Miami (Vertex) at the tune of 13k just for the engine. His engine has been solid ever since but it's not going to be bullet proof.
I'm already on my second engine and if this ones craps out i am probably going with something non-porsche, maybe the V8 Audi one