How about converting a warrior priest from empire or a chaos lord from WOC? Those would be interesting conversions for an Iron Warriors company.
I'm sure u can get rid of any imperial or mechanicus iconography and greenstuff or paint chaos iconography or, better yet, attempt to use some bits off of a warpsmith or dark apostle.The Problem I have with the Techpriest is that is a metal model.. which makes conversions... hard. But I could just paint up as a member of the Iron Warriors and it shouldn't look too bad.