Pretext
PC building is an art in itself, albeit one which is really easy to grasp. What is Minecraft about? Art (i.e indirectly creativity).
A mixture of these two seems like a perfect mix. I'm not sure as to how much this idea's been thrown around till now, but this is something I have been wanting to be added in Minecraft, at least in modded if not vanilla.

How will it work?
Overview:
The mod will be a semi-functional mod. So, how it will work is, that the user collects material to craft PC components which include but are not limited to the case, motherboard, CPU, CPU cooler, GPU, RAM, HDD/SDD. Once crafted, the user assembles the computer through a series of GUIs for the components' installation.

The GUIs:
The GUIs for the components' installation should be realistic. For e.g. Applying thermal compound while installing the CPU cooler or filling the reservoir when doing a water-cooled build. Even though realistic, the GUIs should use Minecraft-like 8-bit graphics.

Use of the PCs:
Once the PC is built, it may be connected to peripherals such as a monitor, keyboard and a mouse. The monitor, when connected to a working PC build, should be used to visit websites in Minecraft (something similar to that in GTA V). Maybe something like a notepad application could be implemented; maybe some sort of game. One more interesting feature to the mod (or feature :P) would be the choice of the operating system.

Marketplace:
There should also be a marketplace where the user will buy the different components. The user will have the option to craft the component by using their own resources or buy the component using some sort of currency, probably diamonds or emeralds.
The marketplace should be available through the inventory in a separate tab and later available through the actual PC.

This might be way too ambitious and also it might not be fit for the Minecraft "extension", but this is something I would implement if I could :) I would include illustrations for the features, but I suck at that.