Hi BL,

When Carrara slows down really depends on your system's memory - but any time you start getting up to 100,000 "real" polygons, sytems tend to slow down a bit. 

I think that you may be setting yourself up for difficulties as the model progress. I'd suggest using a combination of different modeling tricks to keep things under control. For example, the piece you highlighted can easily be created in the Spline modeler yet render with full fidelity. The Spline modeler will create something that is about 1/1000 the memory and poly count yet will render as if it's made up of thousands of polygons.

Another thing I'd suggest is using low poly objects but using Carrara's Subdivision capabilities in the Vertex room. This will yield much smaller file sizes by high definition renders. The subdivision modeler is akin to working with NURBS in other modeling applications.

Ultimately, it's a hobby it sounds like so do what you like. But as long as you are learning to model, there are some other good modeling tricks to pick up. ;-)