I've always preferred starting a career in 1939 rather than later in the war.
The Type II U-boat is a very good "warm-up" boat. It's smaller size and crew complement makes it much easier to learn and master the finer points of U-boat warfare during the "Happy Times" era of the war so an aspiring kaleun has a much greater chance of surviving the later years of WWII.
I usually trade up to a VIIB as soon as I have the 2500 renown needed, after about three or four patrols, and then get the GWK engine upgrade after I have the renown to do so because it transforms the VIIB from a sluggish sea cow into a lethal shark, nearly doubling your u-boat's surface speed and making it much easier to intercept merchants or run away from warships closing on your position.
Although many have counseled you to trade up to the VIIB, Deathfrombelow23, I'd say to just go with whatever you like. Although SH3 is a simulation, it is still just a game and the most important feature of any game is fun.
It might very well be fun and quite interesting to play out a u-boat career in the Type II for the entire six-year run of the war (or for as long as you survived). 1943-45 would certainly be challenging, if not downright impossible!
