I'm currently thinking about giving her the legs of a Catachan guardsman. Necron legs turned out too long, but I will probably use shortened Necron arms on her. My other alternative would be Tau warrior arms, but the geometric shoulder pads might look off, and I would actually prefer her to have at least visible bionic arms.
Her hands will definitely come from Necrons and be magnetized: An empty one for the Chord Claw, one holding a Transsonic Razor and one holding a Taser Goad (representing the Phase Taser, just in case). Also, I might build another pair of arms with attached Transsonic Blades, although I don't think they are a great option.
On her back will either be a typical Mechanicus generator pack or a cape (possibly both, magnetized).
EDIT: Scrap that. Catachan legs are too wide around the hips to fit a Wych body onto, and Necron arms just look too fragile.
What I'm now planing to do is to takes some Green Stuff and remodel Wych or Hellion arms to look like Colonel Straken's. For the legs, I'll take those of the Harlequin Starweaver gunner, but replacing the boots with a pair from Catachan guardsmen.