It looks like a nice machine.A few questions; why switch the servos for steppers?Servos are normally regarded as a better option as they operate on a "closed loop " basis and the factory would have specified something of adequate power.Are you quite certain that there isn't a servo on the Z axis?It isn't unusual for a machine to have a pneumatic cylinder on the Z axis to take the vertical load of the moving assembly.What seems a bit more mysterious is what moves the X and Y axes or were the servos removed for transport?

The vacuum system works on most machines by moving a lot of air and using the pressure difference between the top and bottom surfaces to hold the workpiece in place.Some people use a neoprene gasket to seal round the periphery of production runs but your machine may work as is or it might need a spoilboard on top.Something like a piece of MDF or LDF will be porous enough to hold the workpiece in place and it gets better if the edges are sealed with a couple of coats of sealer.I find it a good idea to use brass screws to fasten a spoilboard in place as they wont hurt the tool when the inevitable happens and you cut a bit too deeply.

In essence,why don't you get the machine connected and see what it can do?Its a lot easier than buying a stepper kit that might be of lesser quality than the hardware you already have and which will need adaptors for mounting the steppers to drive the axes and you won't have to go through the calibration process with a control package that you will have to learn from scratch.It would also be a big bonus if the machine came with any manuals,the learning process would be hugely eased.

Is there any means of connecting the control box to a computer or network?At some stage you will need to program the machine and its much easier to generate a program on a modern computer and load the Gcode to the controller than it is to sit there pushing buttons for one line at a time,The challenge may be that a machine of the age of yours might not have a huge memory and you may well find yourself wanting to do things that require more memory than the controller has.If this happens you may have to find ways of drip feeding the code.