Re: Newbie... what rifle?
I'll throw in my vote for the Sendero. Also, the Savage long range hunter is a good choice.
As for the 300 on TX whitetail, in my experience a .30 caliber 180 grain Accubond does about the same damage as a .270 caliber 130 grain ballistic tip. I have used my 300 wby on whitetail hunts when there has been a chance of shooting nilgai on the same hunt. At first, I thought the 300 was going to do too much damage, but that wasn't normally the case. And I was able to have the better bone breaking power to take down nilgai before it got to the brush.
However, I will admit that if you hit solid bone with a hard hitting 30 caliber bullet, you may end up with a large hole on the other side.
So 7mm mag, 300 win mag, or 300 rum will all work on Texas deer. Pick one and have some fun.