Hi cain
You start of with the VIIA and then upgrade to a VIIB.
Post #63 How Do I Know Which U-Boat I Have?
You will receive upgrades depending how you get on in the game and time in the war.
If you prefer to start with a VIIB then you can use this mod.
New_ Start_VIIB > Start Your New Career Replacing The VIIA
You can find further U-boat types playable in the download section.
Peter