Congrats on the fulfillment of a dream that's awesome! A NULA in .300 Mag not only sounds like a tough one to shoot well, but also one that'll be tough on scopes.
For me personally I like a lightweight scope on a lightweight rifle. Also, on a hard kicker a stout scope is a must. The best way to get both lightweight and stout construction is with a fixed power.
I also just got my dream rifle... A Kimber 84M Classic Select in .257 Rob. I put a VXII 2-7x33mm w/ LRD. This is a rifle I'll shoot out to 500 meters for steel & 350 yards for game. That is my maximum effective range with that rifle/scope combo and if I wanted to go further than those two distances I'd get a scope with dials.
If I were you I'd start out with either a Leupold FX3 6x 42mm w/ M1 target turrets, an SWFA Super Sniper 6x which has turrets, or if you're really convinced you need a variable a VX3 3.5-10x 40mm w/ M1s. Those are all tough scopes with reasonable weight.
Once I had that rigged up I'd work my butt off to become proficient out to 600 yds. I found that 600 yds is MUCH more than 2x as difficult as 300, especially if wind is present at all. That's true of medium/heavy sporters... Add in a lightweight rifle and you've got your work cut out for you. If I could feel confident out to 600 I'd consider how seriously I wanted to go further and whether that rifle/scope was the vehicle to get me there, and adjust from there.
That's my free advice; good luck with the new acquisition I hope it's all you've dreamed it would be!
