Longer range foxing calibres don't come better than a .243, what is this dedicated nonsense, are you suggesting you want to buy a 223 or 22-250 SOLELY for foxes.
The 243 will do it all, better than either of these, and if you are looking at close foxes then a rimfire or shotgun will suffice.
Spot on you will not find a better foxing calibre than your .243 pal, it will, as has been said do it all, .22-250 is a fantastic calibre, but bif you get .243 you dont need it, use 80gn bullets and you can smack foxes and drop deer with the same rounds, as I do, 80gn Sierra blitz under 45grns of