Batman before the new 52?

Batman and Detective Comics have both been going for over 70 years. Even if you were able to get them all, you'd probably be dead before you got through them (I may be exaggerating slightly).

Generally speaking, most comics from back in the 40s are really only worth having for their historical value (apart from Will Eisner's The Spirit, which was decades ahead of its time and still remains a thoroughly entertaining read), so I wouldn't worry about trying to get the earliest Batman stories. Although they are available in collected editions, should you so choose.

If it was me, I'd be looking at starting with the stories from the early 70s, written by Denny O'Neil and drawn by Neal Adams and various other artists. That's where it started going back to the 'Dark Knight Detective' interpretation of the character, after the more light-hearted approach of the 50s and 60s stories. These are also the stories that gave us Ra's Al Ghul and his daughter Talia, leading eventually to the present era with Damian Wayne (daughter of Batman and Talia) becoming Robin.

Batman: Year One by Frank Miller and David Mazzuchelli is a must-have for any Batman library. And although it doesn't take place in actual DC Universe continuity, The Dark Knight Returns by Frank Miller and Klaus Janson is also essential. Then you've got books like The Long Halloween by Jeph Loeb and Tim Sale.

That should keep you occupied for a while!