Your question is too broad. If you can imagine somebody asking for advice on buying acar and their response to being asked what they needed it for was "i need it to have a big enough boot for my shopping and need it to go really fast"; you could suggest a car costing from 20k to a million pounds. It's not quite that bad with gaming pcs but you get the idea.
In the old days of pc gaming you would just match recommended specs for games to be able to run them decently and there wouldn't be much difference between adjusting the game's settings to high, on both visual outcome and denand on your pc. However, there's a disparity between running games on low resolution/details and, what's now commonly known as, ultra settings at very high resolutions. I guess you could achieve very high settings on most new games with a £500-£800 build. If you wanted a build that could run the most demanding games on multiple screens on ultra settings with 7.1 surround sound etc you would have to spend a LOT more. It all depends on what you'd be impressed with. In regards to peripherals, they can be a huge money sink. I'd recommend entry level peripherals and just replace them if you wish. I used to game quite a bit and i was comfortable with basic keyboard/mouse and a stereo headset totalling maybe £40
Putting together a pc may seem intimudating but it's simple assembly. I've only ever assembled 3 pcs; the first i assembled with very little understanding and took me a whole day. Now though with the internet step by step guides and videos assembling is a piece of cake.
Can't provide links because i'm not on my pc.
In short my personal advice would be ti spend no more than a thousand if you want to play at 1080p and assembly is a breeze.
