For water look at Monsoon's materials and Chipp, on the Altuit website has a water tutorial using Monsoon's materials. Monsson's can be purchased at 3D Commune. You can accomplish what you want, but it takes experience and you have to give yourself time to learn how to do it. I second Peggy's recommendation for the Geekatplay tutorials. You have great ideas, but you need to walk before you can run. Just keep those ideas going and learn how to build up to them. 

With layers you can combine many materials on a single terrain. I very often use 1 terrain for my whole scene and have many different areas on it vertically. This is aside from ecosystems. In Terrapack, Chipp has included documentation and materials for multilayered terrain.

Geekatplay has a tute for an island where they have it separated into four sections horizontally and each is different.

Good luck.