As this project has progressed, it's become apparent that I'm going to need to start modelling props and other parts in a 3D program as the source engine just cant cope with the complexity required in brush form. Because of that, I've spent the day learning Google Sketchup, which seems pretty straightforward to use, if infuriating at times. I think I'm getting the hang of it though, here's my progress so far;
Feel free to download it/critique it etc, do what you like with it. As time goes on I'll post more bits up here as I do them.