I was looking at some schematics and remembered back in the day when I used to autocad. Now I know neither of you are big modders, but do you know how well drawing the models in 2D would work? ie. I draw the top part in screen one, then draw the side, then the front and match it all together, then make the final touches on the 3D view.
I would say the 3d modeling itself isn't really the hard part. The tools are easy enough and developed enough. The XSI mod tool has many features that make it simpler than it would otherwise be - for example if you're trying to go symmetrical, it has a tool that will make the exact same change on the other side. Like, if you want a spike sticking out of your ship, you just turn that on and whatever you do to one side will be exactly duplicated on the other.
So, getting the shape done is not "difficult". It will take time of course depending on how complicated it is.. but really I'd say texturing is the worst 