My first big game rifle was a 7mm Rem Mag & I still have it. It's a great cartridge, but it's almost a clone of your .300 Win. A 7mm Rem is a .300 Win necked down .016". These two cartridges are too close together to be adding much in shooting options. The .264 Win, on the other hand, gives you the capacity for light-fast bullets suitable for anything from Coyote control to Elk but optimum for antelope through deer.
The problem with the .264 Win is that few factory loads are available for it. Do not consider it unless you plan on reloading. If not, drop down to the .25-06 where plenty of factory offerings are available. Last I knew the Sendero 2 was available in both .264 Win and .25-06 so you have the choice. I stopped using my 7mm Rem mag for deer because I was doing too much damage to meat. I went to a .25-06 as a step-down with nearly identical ballistics, that would do a lot less damage to something I was going to eat.
As previously stated you can save 1/3-2/3 the cost of factory ammunition by reloading once you cover the cost of getting started. Reloading is like many of our hobbies, relatively cheap to get started on, but it can get more expensive. You can price basic equipment on-line at places like Cabelas or Bass Pro. To start you will need a press, case trimmer, powder measure, scale, tumbler (to clean cases) and I recommend a chronograph. Kits are sold with most of what you need to get started. You will also need a die-set for each cartridge you reload for plus appropriate powders, primers, and bullets. Reloading makes sense if you will be doing much recreational shooting or if you plan hunting out past 600 yards AND if you enjoy tinkering with loads trying to develop the perfect load for your rifle. If these don't fit you, it makes sense for you to go to the .25-06 & use factory ammunition.
As for the Sendero, it's a good rifle. Many find it too heavy for a carry rifle but few complain about how they shoot. I would definitely add an aftermarket trigger.