You might want to put some texturing on the hull (good tutorial here - it's for photoshop but I'm sure you could adapt for Gimp) and other greebles on the ship to give it some interest? Maybe some decals too?
Depends on the look you want, but I've always preferred a slightly more battered and grungy look to something really clean. For some inspiration, take a look at Bryan Gibson's designs on the T20 forum. You may have to register, but if you go into 'The Fleet' subforum and do a search on: STAT THAT SHIP , you'll see some beautiful hand drawn designs (probably the best I've ever seen for traveller).
In terms of generating accurate orthos, if you mean isometric views and the like, the only ways I know are either to make the ship in a 3d software program like sketchup and view it in ortho, or to draw it by hand, or a combination of the two (i.e. do a rough 3d shape and use it as a guide).
Also, consider doing the design using vector (something like inkscape) which may be easier in the long run in terms of drawing and editing.