What TerryM said - good advice.
The 270Win., 280Rem., 30-06, 308Win., etc., are all good caribou cartridges, but the
7mmRem.Mag. will give you a little more reach/more punch at the longer ranges.
And, if you want something to go west with in the future, the 7mm Mag. is a great elk
and moose rifle as well, and not the least bit too much for antelope & mule deer also. Hard to think of anything it isn't good for. Maybe a little light for big bears (griz & brown). Ammo selection is excellent too. Forget Gander's recommendation. Go with a tried & proven cartridge, the 7mmRem.Mag. The Ruger77 will serve you well also.
I've had several Rugers - no problems. They're good rifles.
As for scope, the Leupold VXIII, 2.5-8x would serve well, but also the VXII, 2-7x.
Good luck on your hunt.