OK made some comparisons with BFA. So normal Nylotha rewards ilvl 100 and SL greens equip-able by lvl 50 are ilvl 93. If we assume that something like that will be added in DF then we can predict that ilvl of first ‘tier’ of DF greens will be at ilvl 262.
If i was you, i would get one toon of each armor type up to 60 before pre-patch event, just in case that there will be some nice rewards.