@Max Simmons a 450/400 3" is a GREAT cartridge. It's also the MOST DESIRABLE double rifle at present as its plenty for buffalo and cats and people aren't thinking much of the extra-oomph for Rhino and Elephant in the current political climate.
Bottom line, you're going to pay a BIG premium for a 450/400. I'd rather suggest that you get a MUCH NICER gun in 470NE or 500NE for the same money.
I betcha a 470NE Heym will come around for $12,000 cash in hand and that would be way better than a Searcy (would never, ever own one...see the thread where Searcy refuses to warranty a super-defective gun of his that he screwed up) or a Sabatti, or even a Merkel.
For $12k you can get a dandy used 470NE Watson Bros, George Gibbs, or Heym if you are patient. The same maker's guns in 450/400 would be $18,000.
Suffer with getting a caliber you don't want and get a much better quality gun, or buy the caliber you really want but overpay and thus you can afford inferior examples.
Tricky decisions.