Re: Caliber choice for first time.
If medium game (deer) hunting is not a factor, 223 is cheaper to reload than 308. Uses less powder and lead per shot.
Case size and powder loads for 243 and 308 are relatively the same. Since the circumference of the 243 throat is less than the circumference of the 308 throat, the blowtorch on the throat is more concentrated on the 243. But, in the same weight rifle, the recoil of the 243 will be less.
If you are just getting started, a 22 rimfire will allow you to learn technique without the recoil/flinch getting involved. Then going to the 223 will be the next step. Or, you could start with a 223 bolt action and use light loads and bullet weights for familiarization and lots of cheap, useful training.
My personal experience with 243 is that the factory stuff is loaded pretty hot so that they can brag about their velocity. I used to get very impressive muzzle flash out of factory 243 loads in an 18.5 inch barrel.
Of course, if you are reloading, you could back off on your powder loads and probably help your barrel life. This would be true with any cartridge. But, as you go for the longer ranges, you will be tempted to go for velocity instead of barrel life.
All of this is IMO and FWIW and YMMV.