Unless you’ve got the discretionary income of Elon Musk or Donald Trump, you’re going about it the wrong way. Forget the Old School classics in the .400-class like the .404J, .425WR, and probably the 416 Rigby. Even if you could score a little-fired custom magazine rifle in one of these chamberings, the ammo and components have dried up into unobtainium status. And in the rare instances when they do appear somewhere in small allotments, they’re expensively priced and disappear quickly, bought up by guys who already have one or more rifles in these calibers.
I might buy a high-end custom .404J but that would only be because I already have one and, after years of purchasing in a less-costly market, am well-stocked on Hornady .404 ammo and much .404 brass and .423 bullets. So for me it would make sense. I wouldn’t think about buying a .425 or .416 rifle today as attractive as the pride-of-ownership factor might make it.
Instead, you should be looking for DG cartridges (and their necessary components for reloading, brass and bullets primarily) that are “commonly available” on the current market.
That’s no doubt a relative calculus, but in my view that’s going to be cartridges like the .375s (H&H/Ruger), the .458 Win Mag, and maybe the 416 Ruger. You can still find good rifles and factory ammo and handloading components for all these rounds on various sites, like the auctions sites or on the classifieds sub-forums of Hunting boards like this one or AR or maybe Nitro Express.