Interesting idea and rather ambitious.You will need to take great care to ensure that the 4th and 5th axis are exactly aligned and that the spindle axis is also coincident.My other observation is that the stepper that operates the 5th axis is projecting and you are likely at some point to wish it wasn't there as it will limit access to tight spaces.You might not have a need to get into limited spaces at the moment but once you have a machine, there is no telling what kind of work might come along.If the 5th axis stepper could be flipped through 180 degrees or moved above the 4th axis mounting plate it might open things up a bit.
On the operating front, will you have the means to calculate a toolpath based on the huge tool length your drawing shows?If you were simply running a 3 axis machine it would be a lot simpler,but as I'm sure you know, tilting the spindle will require calculation of the Z axis location in X and Y to compensate for the angles at which the 4th and 5th axes are operating and these distances will vary with the tool length.This tool length will need to be a part of the calculation.As a fellow Freecad user I know there isn't a module of Freecad that will allow this level of calculation and no post processor either.Which means some other system will have to be implemented. I look forward to watching the machine evolve and appear in the solid.