Have to agree with the above for the most part. If you are going to resize to get your brass then i would say go with the lee fl die for the size down due to the better design for the decapping rod. I have bent a number of rcbs rods on military ammo and trying to neck up brass. It just happens. But necking down brass you should have little to no trouble with almost any brand of dies just be sure to use enough lube but not to much.
As for the brass you would be better to look around and see if you can find some 250savage brass and neck it down as it is the parent of the 22-250. New brass will be hard to find for it but you may be able to find some once fired. If not i would do what Pat said and sell or trade the 243 brass for what you need.