I would like to keep the price under $600 if I can.
You are against used, and want NIB from a dealer? That lowers considerably what size and quality of saw you can get. You won't be getting any 70 cc pro saws for 600 bucks new. On the classifieds here, several good deals just hanging around with decent pro saws. Me, with that budget, I would take the best deal and then get it ported by a pro here. You will come out close to 600 and have something pretty darn decent.
OK, new, you can get an echo timberwolf for under 500 bucks. That's 60 cc pro construction. Get it from a dealer, not home despot, and insist it is full richened up before it leaves the store. it is suited for an 18, and would run a 24 with a sharp chain, very occassional use, and not leaning on it all day long. Then take that 24 back off and put the 18 back on. Buy the mix oil from them, tell them you are only going to run high test (no ethanol if you can score it) and if it needs to come back for service they better not give you any grief and tell you you straight gassed it. Save all your receipts, including the gas station receipt, save the empty mix oil bottles you buy from them, and don't run old mix if all you can get is ethanol contaminated fuel. Two weeks max and shake the crap out of the can before you fill the saw tank.
I wouldn't buy any new saw today without telling the dealer that. I do not trust modern factory carb settings at all, no I do not. It is realistically tuned, or..I don't want it, or I would be buying it knowing I didn't care about any warranty as I would be making sure it was properly adjusted and didn't have some smogged out crap muffler on it. which voids the warranty then if you make that stuff work as it is supposed to work, catch 22.
if this requires back room adjusting, that's what it takes. (AT and mtronics not included, not much you can do yourself there other than proper starting procedure, cold and warm)
Good luck with your choice and have fun with the big wood!