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The 8020 is pretty easy to put together, it comes pre cut so it's just a case of putting the fixings on to build it up, you only need a spanner or allen key.
You could seek out 4PLAY Racing on eBay, they're not from the UK but I'm sure they would be up to spec you the rig you want. I'm not sure of any iRacing forum members who would offer that service.
Priced up a rig from the plans you posted (Kens Corner 8020 rig). Worked out at approx £700. Add a good bucket seat (£300), seat fixings (£50) to that plus 8020 for a triple monitor stand (£200?). Add a few extras for pedal and wheel fixings, say £100. I reckon £1350.
Got a quote from JCL for a Just Be Faster 8020 rig in lovely anodized black. £2000 Does however come with freestanding triple monitor setup, fixings for OSW and Heisenkfeld pedals, and a nice Sparco Rev seat.
Tough swallow for a desk/rig at 2K! Worth £650 over sourcing my own? Probably. Needs some further procrastination.