For current XT hubs, the M785 is the current 135mm OLD hub, this doesn't come in a non-disc version, but is supplied with a cap for the centerlock splines, so if you don't fit a rotor, the area is covered. 105 (5700) are only currently available in 130mm OLD, so won't suit your requirement, would expect 5800 to have disc hubs in 135mm.

For having a disc hub on a non-disc wheel, there is no issue with this, and it does give some future proofing, for 6 bolt vs Centerlock, both are good systems, the Centerlock is easier to work with, as it only requires one bolt to be tightened rather than 6 for 6 bolt. As Centerlock is a Shimano design, would expect this to be around for many years to come.