They are a major cog of the imperium. They are the adeptus mechanicus. And i think they are quintessential to what makes 40k so grimdark.
There are two tech priest official releases, Magos Delphan Gruss and Techpriest Tezla. And both include the same torso and robed legs, so mainly it is the heads and arms that differentiate them.
Both kits are versatile and the parts can mix well with other minis in the inquisitor range, especially the head, torso and legs. Frustratingly both kits come with a left arm with limited kitbash usefulness. Gruss’ left arm is a drill, and though iconic, looks repetitive on other models. Tezla’s left arm turns onto his console, so it is difficult to kitbash this bit to him holding a weapon. And the most fiddly bit with the kits are the mechadendrites (those tentacle-like bionics). These are the first thing to fall off when the mini is knocked about too much. And i could never work out why the robed legs were cast in two pieces.
Anyway, here are a stack of Adeptus Mechanicus from a variety of manufacturers or other sources.
The adeptus mchanicus is an opportunity to create some truly weird and grimdark 40k conversions, but i’d suggest then moving away from using too many bits from gruss or tezla.