Simon,
No real gotchas, it is a straitforward conversion.
Fit new, modern steppers - size 23, 4 wire is easiest.
Break out board with spindle speed control e.g. CNC4PC C11, if needed.
5V dc for an active breakout board.
24V dc for emergeny stop circuit (and limit switches and safety interlocks if needed)
Power supply for Geckos, 28V works, more volts for higher speed.
If your turret has tool position monitor, you will need more I/O. Not realy needed, you can use it open loop easily.
Smoothstepper is looking promising for more speed and lots of I/O for panel switches.
Ian