Yes, you can neck .308 to .243 easy enough but you will likely lose a few cases if you do in one step. If you have cheep cases avalable that will hardly matter but you'll have better luck if you can borrow a 7-08 FL die as an intermediate step. And, if you want the necks to last very long they will need to be annealled - easy enough to do but you will need to know how to do it correctly. I'm not going to do a write-up for it but do a web search for "case neck annealing"; you'll find plenty of info. Perhaps the main thing I can tell you is to NOT heat the necks to an easily seen red glow, it's not unsafe but the necks will be dead soft and that's not good for accuracy!
Lee's "Challenger" press is made from a high grade aluminum alloy, it's intended for "normal" reloading chores and it's quite good for that; reforming cases isn't quite normal. It might do fine for the modest reforming you want to do but I'd be a lot more comfortable using Lee's iron/steel "Classic Cast"; it's only a little more expensive and fully as strong and durable as other presses sold at much higher prices, it has some excellent and unique user features and it's very precisely machined on computer controlled equipment.
Unless you have an autoloading .243 you will be using in frequent shooting matches you'll likely never need anything faster than a single stage press. I suspect 90% or more of all reloaders start and stick with a single stage - I have, for decades.
Disregard any 'quick die change' bushing gimmicks. We shouldn't lock dies in a press tighter than by hand and you will soon be able to screw swap the dies in a minute or so anyway
