As it stands at the moment (in the UK), the individual GPUs are so expensive, a pre-built from overclockers is probably your best bet.
Plus you won't have to faff around with building it. My go to references for bang for buck is
https://www.logicalincrements.com/ as you're guaranteed all components are compatible. Most of the links go to Amazon, but you can usually knock a few quid off as most mobo & processors can be bought as a cheaper bundle.
Where pre-built is better is that whoever you buy from can get cards direct from Nvidia meaning the prices are better. Allegedly.
https://www.overclockers.co.uk/ocuk...en-7-5800x-geforce-rtx-3070-ti-fs-1dw-og.html looks decent
You know your budget, if getting pre-built get the best card you can, personally I'd feel uncomfortable spending more that £2k on a machine (as I don't think the extra cash makes enough of a difference) but that may not be the case for you.
Even with a lottery win, I probably wouldn't spunk £30k on a PC (like the most expensive from the OC site), I'd probably go for the 4.5k monstrous build from Logical Increments!
Whatever you choose, please post the build here and I'm sure the more technically minded will help you choose between two!