I've got a set of mechanicum priests and classic servitors which are, as far as I know - very limited and most of the very few dedicated mechanicum miniatures.
However, do note that most imperial armies have models which are drawn from the mechanicum and or that can be used to represent mechanicum style units.
Imperial guard and space marine tech-priests/marines are two obvious examples, but lets not forget the inquisitor retinue models, which include all manner of servo skulls and data recorders etc - plus things like the electropriest.
You should probably read "Mechanicum" if you have not already done so. Plus, "Fallen Angels" also has some mechanicum in there too... finally look into the inquisitor rulebook (if you can get hold of one) as it has lots of tid-bits about them which you may find useful.