One gun? A modern rifle in 6.5 X 55 is an ideal rifle to go from varmits to elk. Especially if you handload. You can go from bullet weights of 85 grains on up to 160 grains. There are some wonderfully constructed bullets out there for almost any job you can imagine. It can be loaded pretty hot in a modern rifle, but modest speeds and slow burning powders generally give fine results. It can be a truly accurate round.
If you don't handload, I'd suggest starting with a .243, and add a 30/06 to your battery as soon after as you can afford it.
Or, buy a used .243 and a used 30/06 and you'll probably have enough to get decent glass.
But, I love my 6.5 X 55's, all 5 of them!