Hey
All depends on your budget really and what you're wanting to play with it. Getting a prebuilt gaming PC from a shop is much more expensive than building one yourself. If you do build yourself then you need to make sure every component is compatible (certain processors and graphic cards fit on certain motherboards, age of item from release etc). Radeon is a manufacturer for certain graphics cards.
Storage wise, SSD is better than HDD for flawless running (OS and games) and you do need at least 1TB space minimum to make it worthwhile. Can pick them up for £30-40. Ram memory will need to be at least 8GB.
GPU and CPU are extremely important for gaming, unless you have no intentions of playing new released games.
New / secondhand
CPU - £100-£600 / £40-£360
GPU - £400-£4000 / £180-1500 (sometimes older are worth more than newer - market fluctuates a lot).
SSD - £30-100 (wouldn't buy secondhand singularity in case of corruption)
RAM - £40-120 /£15-50
Cooler - £10 (watercooling is a lot more expensive)
PSU - £25
Case - £30-200 / £20-100
Peripherals (monitor, keyboard, mouse) - £350-600 / £150-300.
We all use nvidia GTX 10 series (5-6 years old) with 7th gen i7's (5 years old). Age of things aren't that important, but how it's used is. People overclock CPU's and mine with GPU's which burns them out