If you have the money forgeworld has released a magos model that would work well for dark mechanicus:
http://www.forgeworld.co.uk/New_Stuff/MAGOS_DOMINUS_WITH_RAD_CLEANSER.html Head:
There is a half bionic head in the space marine tactical squad box.
Also
GK terminator have a techy looking head in the box (used in the picture below)
Puppetswar also has some rather cool looking heads
http://puppetswar.com/product.php?id_product=18 http://puppetswar.com/product.php?id_product=166 Also making a hood out of Greenstuff is an option. There is a part in this tutorial how to create cloaks and hoods in
GS:
http://www.bolterandchainsword.com/topic/68547-modelling-green-stuff-templar-style-robe/ Servo Harness:
Necron legs work decently for servo arms This model for example used them.
What also works for more tentacle like servo arms is using a sturdy wire. Just bent it in position and add chunks of a plastic rod with a hole onto it. A plastic lollypop-stick works for that.