For the big nasty bovine, a .500 440gr bullet at 1400fps has shown to do everything that needs to be done although still the handgun isn't a stopping gun. It'll kill and it'll definitely change the plans of a big animal, but it's not exactly a stopper.

That doesn't keep the fellows from handgun hunting big bovine, big ungulates and even hippo.

Although anything over the 50/500 caliber is regulated as "destructive device" for handguns here in the USA, if money wasn't a problem I'd go forward with a 20bore or 16bore using 70 or more grains Black or a nitro energy equivalent.
Excessive velocity will only give the hunter greater range. That's not needed for this kind of gun.

There are some handsome hammerless designs - but I'd want small curved back action or island locks with hammers.