I'm 100% with segara82 - it's the exact same stuff I've read and "grown up" with as far as 40k is concerned.
As for the Techpriest ... here I would aim for a compromise. The player apparently doesn't know or doesn't care a lot for the Mechanicus background, its practices, and regulations. Which is always a bit of a problem with such classes, if the other players and/or the GM know better. However, it is not impossible that this Techpriest might simply be an "extremist", a radical, or even pursuing heretical ideas. Invention of entirely new gear is frowned upon in favour of modifications and reverse-engineering of schematics gained from Holy STC prints (which mean restricted access, and no random Enginseer in the field would gain the necessary data "just like that"), but that doesn't mean it never happens. Sometimes the results are a power play that make the bold Techpriest rise in influence over his peers, other times there will be investigations by the Inquisition and the AdMech itself, and dire consequences for any perpetrator who doesn't manage to flee and disappear in time.
Ultimately, the character him- or herself has a free will and can attempt to break the doctrine of the Mechanicus. The only thing that matters is that the player, in addition to having access to all the required resources and knowledge, is aware of the possible consequences of their actions.
If your players are currently fighting the war against the Dominate, you could even turn this into a plot and force a decision by your group that rests entirely in their own hands, rather than being railroaded by your efforts to keep everyone in line:
The Severan Dominate is clearly in a desperate situation and has begun to undermine many Imperial core tenets in an effort to maintain an edge over its many attackers, and when high treason and consorting with xenos are amongst the dark deeds of its ruler, why not add tech-heresy to it?
Perhaps there is a group of rogue Techpriests who started out just like your player, but had no chance to pursue their ideas until the worlds of the Dominate broke free and the Duke began to remove various Imperial Adepts from their positions. This shift in power could have enabled these young, ambitious Techpriests to approach the government with offers of constructing advanced weaponry for his forces, in exchange for any resources deemed necessary by the group - ranging from simple raw material and restricted data caches to slave labour and human test subjects. Think of the many superweapons of WW2 - from ballistic missiles and landcruisers all the way to experimental heliobeam and sonic cannons or the atomic bomb - and the often ruthless efforts with which they were constructed and tested, and you'll get the idea.
Then, have your players come into contact with this group, perhaps being assigned to raid one of its facilities (laboratory and/or labour camp manufactorum), just to give the Techpriest an opportunity to see the possibilities, and then have his renegade brethren attempt to coax them into switching sides.
Ideally, this will lead to an in-character argument between the characters, and an ooc discussion between the players. If they do run over, the squad would become traitors to the Imperium, but fighting for the Dominate should grant them much the same gameplay experience as fighting for the IoM, with the added benefit of the Techpriest now being finally free to pursue their heretical ideas. If, on the other hand, the squad resorts to categorically deny this opportunity, feel free to have Imperial authorities continue breathing down the TP's neck whenever he gets ideas. After all, now you have given them a choice.