You sound keen.
All these things are possible, I've done, well lots.
It's a steep learning curve though. RAS released a set of development tools (I'll post a link later - it's 4am here).
There's a Level Editor and a Textures Editor. The TE will allow you to give Zoe dark hair.
Because the original development team disbanded shortly after the game's release, the methods of modding have all had
to be contrived by clever folks who managed to figure it out. I stood on the shoulders of giants.
For making character models you need 3DS max version 2.5. Hard to get, but there are ways.
It's enormously complicated
if you've had no experience, so best not to jump in to fast and don't be over ambitious.