Type VII B/C/etc?

[Krauter]

Hey, as I'm still on my 4th patrol of the war, just wondering when the Type VII B becomes available?
Also, what are the available upgrades?

I see that there are deck gun (I assume to 105mm)
Conning Tower: Donno what they'd change..
AA: (Twin 25mm, 35mm, etc)
Sonar Stack: This I don't get, I upgrade the the one that can change directions, yet the next upgrade is the one I just left behind.
Etc, etc

Not that I don't mind the trusty old U-35, it's just she's becoming a slight annoyance with the single stern toob

Cheers, and happy hunting