The doppler was a good buy for me after 4 miscarriages. I wouldn't rely on that later on if had reduced movements.
You can definitely find it before 16 weeks. In America I am sure midwives check heartbeat earlier than 16 weeks, I read lots about having theirs checked at 9-10 weeks.
I got my doppler at 11 weeks and found baby the day I got it. Definitely not placenta, mine has the reading on it, I think the placenta is a very distinctive noise and know it's not baby. Worth watching some YouTube vids. Plus doesn't register as high. Sometimes it picks up mine, again, doesn't register as high. Once I find bubs the reading jumps up very quickly to around 160. Was 170 earlier on. I have the sonoline b with 3mhz probe xx