New: Motherboard, Heatsink, and PSU

Used: CPU, RAM, GPU, and Case

The majority of components you find on the used market are pulled out of working systems and will be just fine for years to come. I only recommend buying those specific items new so you are guaranteed to get all the accessories that come with them.

I'd get a Steam Deck with that money.

You will have to look at the second hand market for sure, but you can probably get a decent 1080p 80-100fps medium/high build.

Something like (or equivalent):

Ryzen 5 3600

RX 6600

16GB DDR4

1TB SSD (probably not NVMe)

Whatever case

Whatever monitor, even if only 60Hz, for now

Whatever MoBo

Tier A PSU from the Cultists Network PSU tier list

You can most likely get everything used if you buy from a second hand shop that has a form of buyer protection (eBay and similar) and thoroughly check and clean and test everything you buy

Hmm at this budget you're likely shopping for deals. Mb cpu combos. Basically stuff bundled together. That's likely to dictate what parts you get to buy.

At that budget you are limited to a second hand gpu, possibly something like a 3070 if you're lucky. Maybe get like a 6700XT. Ryzen 5 3600X, 16GB DDR4 and a simple motherboard.

definitely used at that budget, local if you can, go for older gen cpu to leave room for gpu, like 8th gen intel or ryzen 1600, with a 2070 or even better

600 British pounds is ~771 USD If only new parts were picked, following the https://www.logicalincrements.com/ guide good tier

GPU: RX 6600 $200

CPU: i5 12400F $148

HSF: Peerless $35

Mobo: ASRock B660 Steel $120

RAM: 8GB DDR4 $24

Hard Drive: 2TB $52

M.2 SSD: 512GB $42

PSU: SeaSonic 500W (B) $60

Case: Pure Base 500 $80

Personally, I would change this to get rid of the Hard drive to free up ~$50, putting the money into a higher capacity M.2 if needed

I think you would probably be looking at 1080P gaming, and I don't think this built will be super upgradeable (DDR4 RAM, Intel CPU) but I think you would be able to play games at a reasonable framerate

CPU 3600, Mobo b450m, GPU 6700(xt), RAM 16Gb 3200mhz

Something like this: https://de.pcpartpicker.com/list/WC2VTY

Comes out at 637€. A little over budget, but that's also all new parts. It's definitely enough for 1080p, often 1440p even.

To get the most out of the budget, I'd definitely be looking at the used market.

Definitely used 3600 for CPU. They go for very little and will pair nicely even with 3070 class cards.

Used 6700 or 6700xt for GPU, but since they aren't as robust as CPUs, do pay closer attention to damages, dust, performance compared to benchmark, and etc.

Used case, because they don't matter unless for looks. Just make sure it allows for air to pass through.

Heatsink can sometimes come together with CPU. I'd be spending no more than 20$ to cool this one. (However, PA120 is kinda too good to pass on, despite being overkill for the CPU).

RAM may as well be new, they aren't very expensive. SP Xpower Turbine, Ripjaws, or Kingston Fury 2x8Gb 3200 or 3600 are all good options.

Motherboard, wouldn't buy used. And I wouldn't be buying the lowest chipset either. b550m DS3H would be reasonable here or b450m ("m" means mATX, which are smaller than non-m, and usually cheaper), depending on if you need the extra features of b550 chipset.

Power Supply should never be bought used, look up tier lists, and go with middle range one for reliability. 550W and over should be enough for the build. Try not to cheap out on these.

Storage, also never to be bought used. Look for M.2 NVMes. SN770 is best bang for the buck imo, but even Kingston NV2 will do fine. 256Gb is enough for OS and some additional software. Imo, that's too little. Get at least 500Gb. Ideally, 1Tb. Mind you, you could always add more later.