Welcome to the boat mate.
From the sounds of it you are interested in a challenging game experience.
TMO should be just what you want but you might go with the newest 2.0 version. It's very playable but challenging as is, nothing needs to be added.
RSRD is a very good realism mod but you need to know your history and where the convoys and big battles are. Takes some time to learn this.
TMO traffic layer is good to start with, than maybe go with RSRD later.
The mission assignments are nothing to brag about in any case, I just go with them start a patrol and than freelance after completing one or two.
If you have Vista or Win 7 some tips.
Don't put the game on Programs Files. Make a separate SH 4 folder.
Run everything as administrator.
Launch the game with SH4.exe not the default GU.exe. with any OS.
Fred has all the info on radio mods in this thread.
http://www.subsim.com/radioroom/showthread.php?t=119668
Magic