I would recommend buying a $30-60 one to start out with for a few years. You really have to train yourself not just what to listen for, but also how to listen. I know that sounds odd, but there are things about using a stethoscope that aren't easy to describe over an Internet forum. For example, holding it certain ways will allow for less noise interference, pressing down in certain ways can have different effects...
If you learn on a stethoscope that isn't amazing, then you will be able to use a wide range of stethoscopes with great confidence.
Someone gave me this same advice (which I followed) when I started out, and while I could afford a nice one right off the bat, I'm glad I didn't get one. About 5 years into EMS, I bought a Littman Cardiology III, which is awesome, and I do feel that I get a beneficial edge out of it now. However, someone can toss me a $4 stethoscope during a trauma or something and I can use it well enough to get the job done at the moment, a skill which I would not be able to do if I had only ever used nice stethoscopes.
Additionally, there isn't much difference in stethoscopes until you're talking about advanced assessment skills: something I developed after I'd been a paramedic for a while. Meaning, if you're just taking a blood pressure, or listening for generic epigastric sounds, or simple bilateral air movement in the chest, you won't really experience anything different with any type of stethoscope. Once you really understand the high and low sounds in all lung lobes/fields, how to listen to different portions/sections of the heart or abdomen, that's when an expensive stethoscope really shines.
Instead, spend your money on: books/classes after initial training, college courses in things like anatomy/physiology/pathophysiology/microbiology/nutrition/psychology, a nice (but not necessarily elaborate) pocket knife and a nice but small flashlight (one with a low enough setting to be used as a pen light, and high enough setting to be used for searching/signaling/self defense. However, none of these things are necessarily crucial either. Just an opinion of what has helped me the most. When it comes to stethoscopes, your "software" skills before you go buy great "hardware".