Wow, haven't heard from them for a long time...
There was an =][= scale model -
http://www.games-workshop.com/gws/catal ... GameStyle=Of course, whether the painting matches the fluff is another matter. I do seem to remember something about different coloured sashes representing different grades, but I may be dreaming that.
About the Navis themselves, well I know that they pre-date the Imperium (meaning that like the Mechanicus they aren't actually part of the Imperium)& that they have Great Houses which were always bickering with each other. I assume that's becuase they are paid for there services by the Imperium & Rogue Traders (but I don't know for sure). They have a third eye to see the Warp currents through, which they need to keep bandaged up when mingling with the unwashed masses as a single look into it can kill (then likely result in the unforunate Navigator being burned as a Witch).
I'm not sure why a navigator will be on a 40k battlefield though. From all accounts they are physically rather weedy & their psychic powers are pretty specific in getting ships through the Warp rather than offensive/defensive battlefield powers.
IIRC Jaq Draco had a Navigator in the Inquisition War series (although it may have been Eisenhorn & I'm getting mixed up).
Gah, I always think of Lexicanium late - my memory is fairly close, but be warned that Lexicanium is often not the best refernce tool as areas are prone to be wrong...
http://wh40k.lexicanum.com/wiki/Navis_NobiliteEdit: Damn you Ninjas - Damn you to hell!!!!!!!!!!!!!!!
Thanks for the pdf Timewizard - saved!