My guess (and it's probably as valid as anyone else's guess at the moment) is that you'll be looking at 80% epics, 20% ilevel 200 rares to make any impact on the place from the get go. It's going to be challenging.
That said, there's a number of craftable and purchasable epics out there, and we're getting better at Naxx every time (it's not like Kara where we hit a cockblock who held us up for weeks - we've fully cleared the place and I fully expect to be able to do so in quicker time (though the time on the last clear was pretty awesome), so a couple of runs through there will help gear most people up (if only because it's an easy source of a good number of Emblems of Heroism.)
I'm still hopeful that the resets for VoA and OS will be brought down as well - it's all well and good having it so that you can PUG the place but if you're only able to run each once a week in each mode it's a pain.