If it's being supported by something thicker undereneath, go for something thin just out of simplicity. I have thick stuff here, and it's a bitch to work with.
As far as the pattern goes, there's actually sheets that are specially designed to have "deck plating" patterns and such, so before you try doing it from scratch see if your local hobby shop has something close...
Doing it by hand wouldn't be terribly bad. Tedious perhaps, but thin styrene cuts like butter with an X-Acto. Just be very patient, very careful, and when it comes time to sand your cuts be careful and use a very fine grain sandpaper. (Finest you can find).
Working with Styrene's not hard, so don't fret... Also, if you're buying just plain sheets, don't bother with a hobby shop. Find a distributor in your area of large sheet styrene... GE Polymer Shapes (A division of GE) sell ABS Styrene by HUGE sheets (like you need a pick-up truck to haul them huge), but for $50 or whatever you get two huge sheets that'll build you a Death Star or a dozen if you wanted.