Well, you have some idea what is involved and have set yourself an achievable first goal. Good luck.
I've always thought the best subsim would be creeping around a darkened room with an old fashioned steel dustbin on your head looking through a tiny eyehole, trying to avoid someone else wearing a blindfold and wielding a cricket bat.
SH3 has hundreds of things wrong with it, and after a while you tend to forget the thousands of things right with it that are why we are still playing a 20 year old game.
For me the things that really let SH3 down late war are:
- No shallow water (and cold water) acoustics
- Much too effective homing torpedoes
- Aircraft equipment - mk24 mine and sonobouys
- Escort AI - no equipment or tactics against GNATs, no group tactics, no patience
Given the above the type XXI in SH3 might as well be a flying saucer
Fortunately the game kills you in 1943 or early 1944 so this doesn't matter greatly.