You can remove range from the equation by selecting Unlimited Fuel from the Realism settings. Just pretend you met up with a floating Esso station - a covert German tanker or a generous u-boat.
If absolute survivability is your primary goal, you take the IXB because the type VII boats are not available with the 10th flotilla, although if you are driving a VII when you transfer, you might get to keep it (but all type VII have a 6,500 nm range at 12 knots. The tanker will have to become your personal tag-along. However, at the cost of 2 extra seconds of diving time, you can get the huge range increase offered by both the IXC and IXC/40.
IXB:
dive time =
35 seconds
range =
8,700 nm at 12 knots
IXC:
dive time =
37 seconds
range =
11,000 nm at 12 knots
IXC/40:
dive time =
37 seconds
range =
11,400 nm at 12 knots
IXD2:
dive time =
42 seconds
range =
23,700 nm at 12 knots
Although crush depths were different historically (and many boats exceeded these and still returned to the surface), the game does not model these; all type IX boats are 165 meters (all type VII are 150 meters). You can alter these by editing the appropriate .cfg files.
Caveat: how many boats exceeded the above crush depths and did not return to the surface is anybody's guess.