Re: 300 win mag scope?
A 4.5 -14 Leupold would be an awsome 800 yard scope. Low enough power to drag through the bush and plenty of powere when you need it. It is up to you weather or not you go with a 50mm objective lense, but FOR SURE go with a 30 mm main tube and the turrets!!
But, before you buy the scope, I would make sure you have enough money for a quality range finder. That, my friend, is more important than the scope for long range shooting.
Just my .02.