I volunteered at an endurance ride last summer and had terrible trouble finding the heartbeat with a stethoscope. The more experienced volunteers recommended going to a medical supply shop and buying the best one you could find there, avoiding any of the lightweight options. Even borrowing one of those high powered options from another volunteer, I have to admit I still struggled with consistently being able to find the heartbeat, even on horses that were pulsing down from the intense exercise of an endurance ride. My trouble seemed to be distinguishing gut sounds from the actual heartbeat. Since you're interested in keeping track of your own horse only, it should be a little easier- you'll be able to play around with him until you're sure you're actually hearing the heartbeat and then you'll learn where to place the stethoscope on him specifically- turns out that placement varies widely from horse to horse.
You're smart to get one though. Will be curious to hear what brands folks recommend as I'd like to get one to practice more with my own horses.