Darth Smeg said:
While I see the reasoning, that would make it very easy for a Tech-Priests with sufficient cash to become very powerful.
We've reached the situation in my campaign where the combination of implants, talents and gear make our TechPriest virtually indestructible.
The solution is politics, bureaucracy and techno-babble.
As there are countless different sub-groups within the AdMech, availability (or the speed at which something can be obtained) would have to depend on the character's standing with that particular sub-group. If the character is well-known and well-regarded then sure, no problem. But if the PC has travelled to a given Forge World for the first time, perhaps some of his litanies mark him as different and as such, the tech-priests are reluctant to provide him with such sacred equipment. Maybe they don't refuse him outright but rather they require a complex and protracted interview process in which he is questioned extensively about his reason for wanting a given 'special' cybernetic instead of a more 'traditional' one. Some AdMech may not be happy to gift such technology to servants of the Inquisition.
Then there is also the issue of payment: you can ask the player for thrones, but much better to ask for favours or exchanges. As an acolyte, the PC will be privy to much information that the AdMech would be interested in getting their hands on. Perhaps the gear is available but the 'cost' in information is uncomfortably high: the cybernetic device is installed but the tech-priest remains uncomfortable about having divulged certain sensitive secrets. What if his inquisitor finds out? Would the tech-priest be willing to 'betray' his Inquisitor for the sake of the cybernetic enhancement?
Also, "available" doesn't mean right away: there are forms to fill out which then need to be processed, permissions to obtain. There might be a process by which the tech-priest's circuits are 'recalibrated' in order to ensure the right amount of power distribution ot power the new devices / not overload. Some tech-priests might be concerned exixting cybers will interact with the old one in some sort of dangerous fashion.The technology might be available but will need to be brought in from another Lathe World. This could lead to a lot of waiting. Not to mention: it takes time to operate and add the new cybernetics, as well as time to recuperate from the procedures - time which the acolytes may not always have available.
A good rule of thumb which has worked wonders in our game: the tech-priest is generally able to get what they want assuming they're on a suitable world: few issues for a Forge World, a little harder on a Hive world or 'civilized' Imperial world, with increasing difficulty for other worlds. Depending on what is being requested and where we are, it might take a session or three or more to get the cybernetic. It at least tends to space out the upgrades and makes it more interesting than just writing down new stuff on the character sheet.