In my past experience with the pbp medium, I've just done init as it normally comes up, although I would have my players do this as part of their first post of the encounter.  the same would hold true for attack and damage rolls, although saving throws would have to be done reactively.

Don't worry about the dice slowing things down too much. It will a little, but once we get into the groove of it, it should flow naturally. It's only a minor inconvenience at worst.