There are some stunning Adeptus Mechanicus armies out roaming free on the Interwebs. One that particularly inspired me was Hortwerth's, which uses Codex: Imperial Guard.
His first Adeptus Mechanicus army is displayed here: http://www.warseer.com/forums/showthread.php?t=30799
It's very nice. Tank heavy, with Bretonnian bodies forming the majority of his troops. Well done, and not beyond the realms of possibility to convert.
However one army wasn't good enough for Hortwerth! He then set out to make a second army, sculpting most of the models himself.
It's awesome! The Knight Titans are immense, and the basic models are all just stunning. And, big advantage, you can buy them online here: http://www.shop.microartstudio.com/iron-brotherhood-c-36_43.html. Although of course that means you can't use them in official GW events. And I doubt I could paint them as well as he has!