WELCOME ABOARD!
There are hundreds of small mods that work well with GWX. It all comes down to personal choice, but you can look many of them over here:
http://www.subsim.com/radioroom/showthread.php?t=170090
Only one is absolutely essential, though - SH3 Commander. It's not a mod but an external tool that will add to your career the way supermods add to the game itself.