SORRY FOR MISINFORMATION. I was testing the wrong file revision in this video. The night lighting looks wrong ingame in this video, but it won't look like that when you test it yourself. The prelight values I recommended for the interior _will_ look a bit over-exposed as unfortunately I was calibrating the settings for a different file revision. An interior vertex color value of 120 instead of 140 might work better.
You have to set your texture path as in the video called "08 - 3ds Max GTA texture path". If you don't have TGA-s follow the method I wrote in a comment - briefly you have to mass export the textures as TGAs with Magic TXD
- design own door: sure, I did exactly that - possible in single player: it is possible, but I don't work with single player modding and so I can't really provide insight in how that's done