A three door is stiffer in the same way a hardtop is stiffer than a soft top, more structure, less holes. There may of course be exceptions to this.
As for this car, its tidy as it is, I'd personally not spend too much on it, save your money till when a sporting is more within your reach then trade up, maybe just go with a set of alloys now, they set any car off, and keep playing with your ice, the beauty of these mods is you can transfer them across when you do move to a sporting.
Anyway, good effort for your first car, my first car was a brown Chevette[8D]
Stilo TURBO!!!!(diesel)