It really depends on how much you're willing to spend.
The best 1/6 custom McClanes are built from the MJ Choi sculpt offered nearly ten years ago. It's a bit oversized, but it's still the best likeness of McClane out there. Getting an original wouldn't be cheap, but if you're serious about it, have a look over on customonesixth and maybe post a wanted ad.
If you're looking for something cheaper and you don't object to buying recasts, recasts of the MJ head of varying quality are still plentiful. But they're unlikely to be as good as the original.
As for the body, I would either try to pick up a full kitbashed figure from someone, or wait until you have a head sculpt in hand before proceeding with body research, because like I said, the MJ sculpt is a bit oversized and the recasts were a touch smaller, so you likely won't know exactly what you're working with until you have it in hand. I think back in the day Hot Toys Bruce Banner and Han Solo bodies were popular for McClane bashes, but there may be better options out there now. Seamless male bodies weren't really a thing back then.
This is all half-remembered from years ago, but I hope it helps.
There's years worth of kitbash inspiration in here:
https://www.collectorfreaks.com/threads/the-die-hard-thread.25057/
