Modified Cadians will work as skitarri troopers. Some hoods, robes, ragged clothing, bionic bits, integrated weapons and minor mutations will give a 'Heretek' look. How's your greenstuff ability?
Then you of course have servitors for heavy weapon troopers, you can buy servitors singly from bits sites or ebay or convert your own.
Some necron models may also work very well as corrupted machines of the Legion Cybernetica. The Triarch Stalker without the pilot and cupola (so just the spidery walker thing) and some converted wraiths would work well.
Try searching for Skitarri, or Adeptus Mechanicus armies/conversions through google. There's a wealth of inspiration out there.