The classic market may be where you want to look in the USA. A good combo of Continental or English orgin can still be had at what seems reasonable compared to the new European guns. Either over unders or sidXside. Some of the rifle calibers may seem odd to the US buyer, but are effective.
I have three at the moment. Two "cape guns" sxs, and one drilling.
Rifle calibers are 12.7X44R ( about a 50-70), 9.3x72R (great in the RWS load) and a bottle necked 9.5X56R (.375 bore). The shotgun side is 16 ga in all. All are break actiions. All underlevers. One the 9.5 has a hidden peep in the tang and set trigger. The Drilling is set up with pop up opens and scope claw mounts. The .50 is basic working gun with two leaf open sites. Shoots right on top of the sites with a 400 grain bullet at about 1300-1400 fps. Plenty for most game other than Africa or big Buffs.
The other cartridges are fine for deer and boar. 200 grain at close to or around 2000 fps. That 16ga with proper slug or round balls are a good second shot womper.
I have seen a number in 8X57R or even .303. I wish I hadn't passed up the .303s. I will not the next time.
Regulation at 50 yards seems prettly close with all three. Minute of boiler room for deer or boar size animals.
Ammo can be a challenge with some if you don't handload. Good thing for S&B. Pretty fair ammo and bullets in these calibers.