I just can agree on what steve said.
Drop your plans of making a map with a special theme.
Nobody will contribute a texture or a model,
since you don't even have an idea what gamemode you should choose.
It's tons of work do create a well playing map.
You can thing about a special theme after your 5th map or something.
Because your first map WILL suck.
There has been exceptections, but regular the first few maps are crap.
I know that those are hard words, but I don't want to discourage you.
If anything, I want to help you.
Start some maps with a basic layout.
Try to analyse and rebuild areas of valve maps, to understand the scale and gameplay.
And as soon as you see there is something bad in you map, fix it.
Or if it's a big problem, scrap the whole map and start a new one.
As soon as you learned something from a project. It was worth the time.