Unfortunately I got a bit excited and forgot to take a picture of the first POST screen. Suffice to say that it POSTed and I was very happy and there was much rejoicing.
After that we put everything in and got windows installed. Another very happy moment.
This is actually a fan controller/temperature monitor that runs from 12V pulled from a molex connector on the power supply and (we guessed) 5V from a funny shaped connector neither of us recognise, let alone have attached to the PSU. So I did what I'm good at and came up with a solution. I cut the funny plug off then soldered the wires onto the 5V going to the power light. I checked the current draw on this when we tested that it was actually 5V using my benchtop power supply and the current draw was not registering on the scale, so less than 0.5 mA which I was quite happy to pull from just about anywhere.
The tricky thing about buying parts off ebay is that they may not always work together. The motherboard is from a HP Z400 workstation which looked fine on paper. However initially it wouldn't POST. This is because HP decided to use +12V on some of the pins of the 24pin plug where the ATX standard that everyone else uses at +5V and +3.3V. Not very useful. So I had to splice in this adapter that I already had to a molex Y-splitter so we can get the extra +12V power.
So there you have it, the build is finished and tested. In the next few days I will do another post detailing the parts list, final cost and some benchmarks (also some thoughts on that parts I used) so keep an eye out for those if you are interested.
If you want more detail on how to build a computer let me know. I have another build coming up in the next few weeks that won't be as crazy as this one but will still give me an opportunity to show how it's done in great detail.