I started by deciding which configuration I want to build, afterwards I do research do discover what was the most interesting U-boat to ever sail in that configuration.
I want a configuration appropriate to an era when U-boat were still scoring major successes, but war has already been going on long enough for U-boats to show improvements based combat experience. This means a boat that already had it early-war net cutters removed, but still has its deck gun, and not yet outfitted the pathetically futile snorkel and Antiaircraft artillery expansion of the late war.
I also liked the antenna bulge on the port side of the conning tower. It has combat improvement written all over it. So that rules out the Blum & Voss boats like U-552. So I am building a boat in the configuration similar to U-253.
Now I just have to find out which of U-boats that shared the same configuration as U-253 had the most interesting combat career.