First of all, very very nice. You seem to have acquired some nice high quality components. The 36" of Z seems very ambitious, why do you need so much Z height? foam? Rigidity will likely need the most attention, especially if you want to machine metals. I took apart a similar sized milling machine and the whole column was cast iron about 1" thick and had cast supporting ribs on the inside throughout the column. The servos used were also much bigger (180v dc servos, 3 hp) and to support the Z axis it had a pneumatic brake meshed with the gearing of the Z axis.
Keep up the good work.