To answer a few of the more technical questions:
You're perfectly free to use and modify any already existing drawings for your own ends, as long as you credit appropriately. You can find more information about that here
Generally people aren't very keen on drawing other people's ships for them. There have been some collaborations between some members, and some people let their AUs take place in the same world, but if you want a ship drawn for your AU, you're best served doing it yourself. Something I do encourage you to do either way, since there's no way of getting good at actually drawing ships unless you've practiced.
Regarding where to begin with a design or a drawing; , assuming you're making up your own, decide on what kind of ship you want to draw. Research that particular type, look at real life examples that are similar to what you want. Look at what factors affected their design, and why they were built the way they were. Make a few rough measurements to get an appropriate size, and then start drawing. I usually start with the waterline and work my way upwards from there, finishing with the underwater hull. Get the shape of the superstructure where you want it, place out armaments and stacks and so on, and then sprinkle on details - railing, portholes, equipment, shading, etc, and you're ready to post.
In regards to the setting of your AU: Do whatever catches your fancy. Do appropriate amounts of research for your AU, and then knock yourself out.