Buying loaded ammo to get the brass is not my first choice, but I've done it, and will do it, if I can't get "decent" brass easily any other way.
I'm not much of an order online guy. Probably because I got into guns & reloading long before there was any online to order from. Components were one of the main reasons I haunted gunshows back in the pre-shortage, pre-panicdemic days when we had them.
I have guns (or had) where buying loaded ammo was the only sure and easy way to get brass for reloading. Some calibers don't have a large reloading following and finding quantities of new brass is...sporadic. Finding some loaded ammo is common.
.243 is a piece of cake in that dept. While not quite as common as dirt (and no where near as cheap) there's plenty around. No, buying loaded ammo doesn't help with future load development, other than providing the cases to load, BUT it does do that.
And, it does two other things. It gives you a "baseline" to reference against to see if you can make your ammo as good, or better than the factory stuff.
And the other thing, not too important with many calibers but important in a couple, and that is, YOU KNOW how many times the brass has been fired, and what it was fired from.
Again, not that big a deal in .243, but it is for some of what I have. I gave up on "once fired" brass completely in .303 British. I also avoid "modern" milsurp 7.62x51 brass. Its often fired machine gun ammo, and can take more work to reload even when the cases haven't swollen too much.
Enjoy your .243, its a good round for lots of things, and a great round for a few things. Do get at least one box of factory ammo, to give you a standard to compare your loads against. Velocity, accuracy and
function in your rifle.
As I'm sure you're well aware, semi autos can impose limits which manual repeaters can often ignore.
Good Luck, have fun, be safe!
(not in that order!
)