Hello stellaferox,
as for mods there is an ample choice with SH4, but most of the time you are going to need the upgrade 'U-Boot Missions' v.1.5 which you must aquire separately, this is not a mod but an official expansion to the game. If you have the Steam version you should already own that.
With this installed, there are a couple of big mods that you can install via JSGME:
RFB (Real Fleet Boat) is a mod that focuses on realism improvements. Nothing I can say more about , I didn't play it.
TMO 2.5 (+ OTF) (+RSRD) is a serious overhaul to the game, making it harder with better grafics and many new features like walking in your sub and clickable items (if you install OTF), better optics etc. Comes with a complete manual. Optionally you can add RSRD which changes the campaign traffic.
TMO wTw 2.5 (+ OTF) is an easier version of TMO 2.5, same features but easier gameplay. I would also install OTF with it.
Ralles ModSoup: based on TMO, OTF and RSRD is a compilation of the mods above ready to install with optional mods included.
FOTRSU (Fall of The Rising Sun Ultimate): A mod with many, many extra units including all features of TMO which the campaign is based upon.
I hope this helps you in your decision, have fun with SH4!
Greetins,
XS