Hi,
In principle, there's little difference between a Stepper and a Synchronous motor, but both generally give much lower (maximum) torque and need at least one "H-bridge" (bi-directional) "High Power" driver, compared with a simple, single PWM/FET driver to a dc motor, which the OP has already planned to use, but apparently not implemented yet?
IMHO the issue is not driving the motor, but synchronising the movement of the film in the gate to each instant when the camera "shutter" closes. The OP insists that he cannot obtain this from the camera, so an (inferior) alternative is to assume that it occurs exactly every 20 ms, and to derive a "sync" pulse from a crystal oscillator, or from the 50Hz mains for an initial test. Note that "flywheel" sync is basically just an early version of PLL technology, which is now found at the heart of almost all modern electronic devices (including the PICaxe ! ).
Alternatively, I see that the camera has a "Time Lapse Capture" mode, but it's not clear from the manual if that can use an external trigger (e.g. each time the film is stationary in the gate), or if it uses only an (asynchronous) internal LF pulse generator.
Cheers, Alan.