Hi, did you just use Google sketchup for the whole image or just for perspective guideline and then paint in pts? I've just heard about this software recently, so I don't know if the free version can be enough for such a painting like yours?!
His designs can be chaotic and calm at the same time, I personally refer those who had the old curtiços bunches here in São Paulo Brazil. Congratulations for such dedication and beauty that passes in your art ... And my appologies, i used the google translation...