Alder Lake won't compete with Zen3, at least not for long. AMD's answer to Alder Lake already exists in the flesh - as Epyc engineering samples, and it's reported to have 29% higher IPC than Zen3.
By the time Alder Lake and DDR5 hit mass market, AMD will be selling something 29% faster than Zen3 on TSMC 5nm and I don't know about you but I trust TSMC more than Intel's still no-show for any high-performance 10nm parts. 10nm might be better than it was but there are no desktop, server, or datacenter products made using 10nm. It's flaky and unimpressive - we've had a bunch of Ice Lake laptops and they run hot and slow, two have gone back under warranty for IGP failure. Tiger Lake may be better but once bitten, twice shy so I've never used one.
If value for money is important and your existing rig is 10+ years old, just grab an i5-11400 with some DDR4-3200 on a B560 board. It's going to be a vast improvement over a C2Q and if you grab yourself a decent budget 1TB NVMe drive like the WD SN550 or Kingston A2000 you're going to be getting a big upgrade for not a lot of cash.
I'll assume you already have a GPU, if not stick with the HD 750 IGP until GPUs are available again. Amusingly the GTX 970 is terrible for mining so those are going for a reasonably price on ebay (£125 or so) which makes them "good" performance/$ if you need
something right now that's
approximately capable of 1080p60 in plenty of games.