Your .243 is an excellent whitetail cartridge with the correct bullet selection. I used one for about 20 years and killed maybe 30 or so deer with a 100 gr Hornady bullet. All but one were dropped with a single shot. Now for your choice of a second rifle with the prospect of elk and moose you can't go wrong with the good old 30-06. It will handle any elk or moose and is easily more than enough for deer. Recoil isn't that bad even with a light rifle. There are undoubtly better cartridges suited for these bigger critters if that was the sole intent on getting one but I see no sense in getting a 300 mag or bigger if you are only going to maybe go on an elk or moose hunt. That's my two cents anyway.