I'm on the Black Mesa mod as a prop artist and you seem to wanna work hard on this but let me give you a few words of advice.
Get a writer for help or do all the paperwork yourself. Someone that handles all the needed dialog and even writes down events that are gonna happen. This is essential and maybe even the most important role. You need to get everything on paper and plan the chapters carefully.
Every map should be on paper and carefully planed before you start to map it in hammer. Essentially, the more you have planed the easier others that join your project will have to work on it.
You all can of course start of and map the base for it but since SiN is a storybased shooter allot of research and such is needed. And the story script you have to be quite carefull with. We all have a hint on episode to but after that everyone is clueless.
Here's another fan with the same idea. Put your heads together and we'll see what comes out of this. Good luck.