He should get real and put the price at 40k cut his losses. Its not 2022 anymore the hypes kind of died.... People do want it but not at 50k.
You can probably squeeze a nice M4 comp xdrive, M2, M3 all G series, Audi TTRS, RS5, 2021 RS3, Porsche Cayman 981 GT4 and much more for 50k. As you said C63, AMG GTS 2015/2016. 718 GTS 4.0 if youre lucky. For the price of that Z.
Cars are cheap in UK compared to elsewhere tbh. I mean a Quadrifoglio at 30k, even some nice Aston martins with a warranty at 50k. People in Malaysia and Singapore pay 2-3 times the price. We are lucky somewhat...