Which subs carry which upgrades?
 

I know that upgrades follow a particular timeline. For conning towers, it is very easy to tell which ones you will get based on your sub. For other stuff, like electronics, it isn't so easy. So far I haven't seen anywhere that tells me what upgrades each sub is capable of getting.

For example, if you wanted to, could you have a type IIA with the best radar/sonar/decoys/etc?

I believe the question is "would you want one?" ;)

I know offhand that the battery upgrades are specific to U-boat type, and the deck gun type is dependant upon the conning tower type. But I haven't seen a list of these specifics either.

I believe that the other upgrades are available for all boats, regardless of type. Please correct me if I'm wrong.

When you are in port and looking at the upgrade screen if you hit the ? (Help) button, you get to see a nice timeline of what is available when and to which sub you can add it.

I too would question whether you would want a TypeII with Schnorkel, radar etc. You don't get to go far enough to warrant all the good kit!