Also consider 4 each 32 channel units and put 25 cams on each unit. The 32 ch. units are more readily available but more importantly, you don't lose ALL your cams due to a malfunction on one NVR.
Logically distribute the cams across the 4 NVR's so that if you lose 1 NVR you still have reasonable coverage until it can be repaired or replaced.
You may even consider having one spare NVR in the closet if funds will allow it.