For listening to the baby so early in pregnancy, I dont recommend buying one. The ones that you buy for a decent price anyway, really arent that good. You really cant hear that well, you will have a hard time hearing the heartbeat because they dont pick up that well, can can be staticy. At that is my experience and what I have heard about the ones that you buy.
I rented one from www. babybeats. com. The one that rented is $47 a month. It is very high power, and it is acutlaly the exact same one my doctor uses. Also, I have seen it used on TLC's Baby Diaries. Anyway, I was able to hear the heartbeat very easily adn quite clearly at 10 weeks. The device is strong, and clear. I love it. Nikko_TTC actually has the exact same one I do, and she also loves it.
You can get a much better doppler thruough renting one than buying one, at least for listening in very early in pregnancy, but that is just my opinion.
Good luck, and have fun!!!!
I told my hubby I wanted one to, so I'm abt to start looking into them also.