bulldog563,
Your .500 will do for the big nasties that want to stomp you into a pile of jelly. I think the .375 is the perfect all around rifle. I used mine for deer and antelope hunting here in Wyoming. It doesn't tear up as much meat as my .300 did. So I would say go with the .375 as your medium rifle. It will certainly work for buff. For the small bore I would go for the .243. It works great for critters in the 100-300 lb range. A .243 has very light recoil and they are flat fun to shoot. I have a friend who uses his for elk and they are one shot kills.