I would get...
TMO (makes escorts smarter and tougher as well as serveral other tweaks, generally makes the game more realistic)
http://www.subsim.com/radioroom/showthread.php?t=132330
RSRDC (A new and much better constructed fleetboat campaign), FOTRS (I haven't tried yet but it sounds good!), WDAD (a Royal Navy Campaing!), or OM (haven't played yet but seems really popular, german U-boat campaign in the Atlantic and pacific theaters)...
Improved Stock Environment (greatly improves the graphics)
http://www.subsim.com/radioroom/down...o=file&id=3590
Theres several choices and all pretty awesome!
These two pages are also great places to help research new mods to try...
http://www.subsim.com/radioroom/forumdisplay.php?f=219
http://www.subsim.com/radioroom/forumdisplay.php?f=234
Hope this helps!
Good Hunting!