Caporegime
I had a Ryzen 7 3700X and it was a great CPU. It consumes very little power for an 8 core CPU. If you can get one for close to £100 that is a bargain. Zen3 will be better for gaming but as stated above I would get a Ryzen 5 5600 at a minimum. If you shop around it can be had for closer to £160 if there are discount codes available. WRT to motherboards,try and get something with reasonable VRMs. If you don't mind a shorter warranty you can B-Grade or damaged box B550 motherboards for much less.