Well ya, the equipment would be different between a German U-Boat and a US fleet boat. SH 3, 4, & 5 tried very hard to model the equipment as it was on the real boats, where SH 2 didn't so much. Also how these ships were operated and the tactics used were quite different as well.
You might have an easier time transitioning from SH2 to SH3, that way at least you are still using the same type of submarines and some of the techniques and skills will transfer over.
To be honest, SH2 was not a very good game. A lot of features were broken, and the graphics were pretty bad. That was one of the big reasons why subsim helped fund and create
Projekt Messerwetzer to try to fix the game. I myself could not imagine going back to SH2, not with SH3 and SH4 available. SH3 and SH4 are light years ahead of SH2 in just about every conceivable way.
As for your question, I can't comment on the original SH, didn't play that one. The mod you speak of for SH2 is a good mod for what could be done, but it doesn't hold a candle to what SH4 offers, especially if you get the mods for it. Honestly I think you would be better off spending the time learning how to play SH 4. It is a reasonably high fidelity simulation, and those always take some time to learn, but they offer a far more real experience too. Also our forums hold a lot of resources and information about those games, and you can ask us questions too. A lot of us don't mind helping.