Depending on the age of the motherboard, it may not have the PCIe bus ‘lanes’ or version needed to support the more recent GPUs, so you’re in for a m’board as well - and why do that without upgrading the processor?
I’m in process of building a new PC which should work well with v9 (new case, power supply, motherboard, processor+cooling, GPU). Cost was USD1688, could have been maybe $200 less if I hadn’t also picked everything for low noise. Pretty far from £5000.
I picked an AMD RX 9070 based on this thread Choosing a GPU for DxO Photolab: the answer? - DxO PhotoLab - DxO Forum, the AMD Ryzen 7 9700X as it was the best performance I could get for a 65W TDP (making it relatively low noise).