taylorce1 usually knows what he is talking about and I have always valued his opinions. I'll just add that the military looked hard at the 338 Lapua before deciding on the 300 WM. A new world record benchrest record was recently set with a 300 WSM.
The .300's have something going for them, and on paper at least a 7 mag looks at least as good.
I'd start with the 7 and by the time you get really good at 800 or so with it you'll know what you need to go farther