As always, warscroll change, miniature stay (well, whoops ? But ossiarch, being a pure AoS army, should stay for a long time), so for me the first criteria is always: do I like how it looks?
That being said, its probable safe to avoid buying Arkhan because its a box shared for two armies (and GW does not want to do that anymore so its just a matter of time before the box will not be sold) and because he is dead in the fluff (like, nagash will probably make him come back, but who knows what he will look like). However, the miniature itself (the dread abyssal) is great so if you like it, there will be a way to play it. Its the same base as katakros so you can count as, altough karakros is such a wonderful miniature itself…
As my VDD says, dont buy paper right now.