Zwift on a Budget

In the last month or so I've found myself getting into Zwift, the online cycling game. Last night I attended one of their events, where I ended up demonstrating racing in front of a large crowd (who were mostly looking the other way), and showed conclusively that turning up late to the race start with no wifi isn't a brilliant idea! (Much respect to the colleague who managed to build his computer for me in the 4 minutes before the race started and managed to get me starting only 15 seconds behind the main pack!)
A few people have been concerned about how much it might cost to run Zwift, particularly if you're going to end up leaving the computer in the garage for ease of access. So I thought I'd document my setup to show you can run it on fairly old kit.