The 7x57 is presently one of my favorite rounds, and the round that I'll be hunting with this year. It's got enough punch for most non-dangerous game, moderate recoil, and it'll shoot bullet weights from 100gr to 175gr. It's a good round. You'll not likely find any cheap surplus ammo for it, but it's still not terribly expensive - say, $15/box on average.
There have been number of rifles chambered in 7x57. The 1893 and 1895 Mausers, for starters, along with some post-1898 Mausers and a bunch of commercial rifles. If it's an old surplus Mauser - keep in mind that it's between 100 and 115 years old, and have it checked out before you shoot it. Most decent condition but non-matching serial number '93-'95 Mausers will set you back around $200. For $120 - you better know what you're getting into.