Welcome to the site!
Regarding your decision between the 7mm RemMag and the 7RUM, they are both excellent cartridges, but they are really different. They both have enough power for anything in North America, the RUM can just do it at a longer distance. The 7RemMag will have better barrel life than the RUM. The 7mm RemMag is also very forgiving when reloading.
Your decision will be more about what you want to accomplish with your rifle, as they are both the right answer (but to different questions
Tell us more about exactly what you want to do with the rifle, how far you want to shoot it, at which targets and how often you will be shooting.
How far are you able to shoot locally?
There are other equally good caliber choices out there, the 300WSM is essentially the equal of the 7mm Rem Mag and is even more forgiving from a reloading perspective.
Thanks and welcome to the site,