There is no official word on the harbingers and their look so far, except the base cryptek model and the picture in the codex.
Personally, I find that the fluff entry implies the crypteks to be a lot more varied than the average necron. That, to me, means that it's conversion time.
Statwise, they are closer to a warrior then a lord, so I would advise using base bodies that are not too buff. They are still just scientists, not elite soldiers. Scientists that can scramble brains at range and turn you into base atoms, but still brainy dudes.
Personally, the only fixed point I've used on my crypteks are lithe bodies (warriors in stead of lychguard or lord) and the steel beards, which are easily made from the backspines of a preatorian. After that, I just went with the feel of the harbinger and tried to match that to the model.
I have two finished, with a third
wip, and the other two I have drawn, but not yet built. I should upload some pictures.
If you want some more varied staffs, I'd advise the technomancer staves from Kromlech. They're good looking not-necron standins if you grow tired of warscythe variants.