As with anything in hunting or shooting buy the best equipment you can afford.
On forums you will receive many opinions on what works and what does not and since you asked I will offer mine.
If you are planning a buffalo hunt with plainsgame and want to carry only one rifle a 375 H & H or Ruger is tough to beat. I feel Controlled Round Feed in a bolt gun is a must for Dangerous Game. I have no experience with the Ruger rifle, but it seems people who own one do like them. I have a CZ 550 in a 375 H&H with upgrades from American Hunting Rifles and would recommend it to anybody, however you will spend $1000 more than the factory Ruger. It can hold 5 down and one in the chamber, so it gives you some firepower. Make certain the rifle has quality iron sights and learn how to use them. A good low power scope such 1.75 by 6 or 2.5 by 8 scope will work fine, I like Leupold. Use quick detachable mounts. There is nothing you cannot kill with a 375.
If you want two rifles an upgrade in firepower is never a bad thing. Ruger is offering the 416 but my comments are the same as the Ruger 375. I have a 416 Remington and it definitely hits harded than a .375 and recoil is manageable. The .458 Lott is another option (especially loaded with 500 grain bullets at 2150 FPS) but you have an increase in recoil over the 416. I have a Montana Rifle Company 458 lott. It seems to be a good rifle but I have not put it through the trials yet.
My second rifle on a two rifle hunt would be at least a 300 Win mag, preferably a 338 Win Mag especially if you are hunting where dangerous game is present.
As far as ammo if you are buying factory rounds any of the premium lines will work. I like TSX or Swift bullets.
Learn how to shoot the rifle from position. Get it sighted in on a bench and practice standing, kneeling, sitting, from sticks, leaning against a tree (you get the picture).
Now if money is no option there is nothing like a fine double
Good luck, opinions will vary, find what you like and enjoy the whole experience.