Ok I'm a Type IID junky....so here are my two cents. First off I've been using the type II since the 1st of September 1939. The IIA has a very limited range even a 3-5 knots. (I believe its 3000+ or so) which means you have to watch the fuel or end up off the coase to France with nothing aboard
But the IID's have a much greater range. At 5 knots you have a range (according to my navigation officer) is like 22000km (i'm not exactly sure) so range is not a problem here its all personal preference.
As for your updates. If you enjoy the type II's the battery update is a good idea. You get good range and at slow speed you can stay under water for a long time. The engine upgrade is not a good investment in my book. Although you get an increase in speed (i think its +1kn) you lose alot of your range because of this. I stick with the basic engine and updated batteries.
As for the skeptics in my 9 patrols I've snuk into 6 ports and sank over 60000 tons including a heavy cruiser and an oil tanker that nicely burned or 10 hours.
Jan