Any links you'd like to share? If i remember correctly there was a site where you could view all the textures, that would be very helpful.

I'm also looking for a .pcx thumbnail viewer (codec or similar, so it's possible to see them directly in explorer) for Win7 X64, anyone know if that exists?

Thanks.

Someone else can probably describe this more correctly than I, but from what I know the stripped missions (that come with the game) have their room brushes stripped, so you cannot make any terrain changes. Hence unstripped missions that include this data. As described in the readme not all missions became available unstripped. With the stripped missions only changes to objects are possible.