Normally you can not hear the heart beat through a Doppler until around 12 weeks. As far as a brand I don't know. I didn't get one with my first baby, but I to want one with this baby! Congrats!
The one I have is called angel sounds and it's awesome!!
I have the facelake fetal Doppler it was given to me it's really good. I like it started to hear babys heart beat 10-12 weeks. At first it's hard to find but don't let that worry you. Now that he's moving all the time I don't use it very offten. But it does help give me piece of mind.