If its going to help with peace of mind I would get one. Only thing is a Doppler will only tell you bub is alive not weather it is distressed or not so if you have any concerns I would not rely on the Doppler as reassurance. I have heard of a few situations where women have had false reassurance from a home Doppler and had a poor outcome as they have not seeked medical advice for various reasons so I think its always important to ensure it's not going to replace getting check out if you have any concerns about lack of movement etc.
I got one with my last due to bad anxiety caused from infertility. I didn't buy it until I was 37weeks and had my DD a few days later but I really wanted it for when labour started so I felt more comfortable staying at home during those early labour bits.