lol.
another way of thinking of this is that to make HL2 maps you need to know brushing, meshing and texture artistry. if you are making a super high quality map, which of course all mapcorians do, that requires a lot of work. i forsee that a good map designer-layout expert will design the base gameplay of a map (layout and brushes), work with a good modeller who can crank out required meshes relatively quickly, and work with a 2d artist to create textures and maybe skin the meshes too. of course you can do it all yourself, except it will just take more time.
back on topic,
For now don't worry about how much meshing you need to do and exactly what objcets you need to mesh and what you need to brush. best advice is to just experiment once you get the new engine and SDK and i'm sure you'll quickly get the feel for it. as always, best way to learn is to experiment and just "do" it and don't worry so much about planning your level 100% before you even get the new engine.
oh...this is all speculation. I don't map in HL2 yet so i defer to Zaphod.