I've been obsessed with starting a Mechanicus army lately. I have purchased some Thallax shock infantry and plan on getting the new Magus and Castellax models upon release and I will post pictures of my progress once I get some built. With so few choices I have been checking other model ranges and have found some really nice alternatives. Below is a sample of the Iron Brotherhood minis from Micro Art Studios. http://www.shop.microartstudio.com/iron-brotherhood-c-36_43.html
Next up is from Hitech Miniatures. This range would make a great 'counts as' Grey Knight army with a Mechanicus theme. http://hitechminiatures.com/
Last is a video I found of a beautiful 'counts as' Mechanicus army. My conversion skills pale in comparison, but I still found the video inspiring.