The shader was, from memory, as simple as it gets. Literally just a diffuse map. No spec or normals or anything else. There may have been some lighting from Marmoset, from looking at it it looks like the Blue Sky preset, but im not sure, it has been years since I made this.
Sorry I couldnt be much more help. Generally with hand painted stuff like this, if it looks good with no lights, you've done a good job with it. Other (much much better than me) artists to check out would be Jessica Dinh jessicadinh.blogspot.com.au/ and Faf fafart.blogspot.com.au/