Outside of very specific usage scenarios, you won’t see a significant upgrade in performance vs the R220 running a similar CPU, or a massive power saving, or an ability to add any more RAM/drives/PCIe, so my first question would be what is dropping north of £500 inc RAM/CPU actually going to get you vs the R220 running a similar config? I say that as someone who just dropped a frankly obscene amount on rail kits for 3 Supermicro servers, so I’m not judging you here - we all have our own reasons for doing things that in hindsight we may think were less than ideal. In my case I could have purchased another R720 that I have the rail kits for already and called it a day for about 1/2 the money.
If you do want small/quiet and it is just for firewall use, £55-60 buys you an OK SFF with RAM and potentially a drive (junk it and use a small SSD for power/noise savings, most people have old drives). The £55 AMD option comes with dual onboard i210’s and 8GB of RAM.