Nice machine and extras.
First of all, being that it is all complete. I would try to interface with the existing board via the DB25 on the side, use Linux or whatever will work. Search for threads of other builds. It really depends on your understanding of CAD/CAM/Electronics/GCode etc.
What time frame are you anticipating? The G540 with either the existing internal power supply or the extra one you show could be up and running very quickly with an experienced user. A day or so. Possible with the existing electronics too. Again, that is an experienced (and willing) user.
There seems to be some pretty clear markings on the boards. Might be beneficial in regards to pin assignments etc.