Im saving money now for buying a new gaming PC.
Im one of those people that when they buy something they will buy the best, because im not going to buy a PC every year i want it to be good / great for the close years for i will be actually enjoy this.
You say you are someone that when they buy buys the best. With that in mind I would suggest £7-800.
That would cover CPU, Motherboard, Ram, GPU and PSU. If you need a case or storage upgrading as well, then more.
If you dont currently have an SSD, I would recommend getting one in your next build, it wont improve the FPS of games, but it will help with the load times of the OS and anything else installed on there.
As to buying a Steam Machine, from what I have seen these are rather pricey for the components. I wouldnt recommend them purely for this reason, a home build will be cheaper for the same parts but in a case that isnt as pretty.
I would say a GBP800 build without a monitor and OS will satisfy your needs. If you live in a hot place then an Intel processor is better (i5 3570k) and if not then an AMD one is fine (FX8320 or FX8350). Get a Z77 chipset motherboard for an overclockable 3rd gen Intel or a Z87 for a 4th gen Intel. Go with an AMD graphics card due to better frame rates (R9 280X at least) or if you want an Nvidia one then a GTX770/780. Buy an ATX motherboard as it will be better for SLI/Crossfire. Also, make sure the power supply you chose to get is of a high Wattage and supports multi-GPU. If you can then get an SSD. I reckon 8GB of RAM is enough (get 2x4GB as it's better). Get a case which supports your motherboard layout and PSU and has enough fan slots. If you plan on water cooling in the future then check if that's supported.