I see it's been a while since you posted but maybe this will be of some use to someone else out there. When we asked our Dr. about the heart monitor he said "Don't get one unless you want a 100% chance of ending up in the hospital," as in, the heart monitor will undoubtedly not work right, and when you are unable to find your baby's heartbeat you will panic and be forced to rush to the hospital.
Well, when he told us this we actually already had one on the way in the mail. We didn't rent, we actually bought a cheap one one that was about 100 bucks. After speaking to the doctor we decided that we weren't going to use it for "monitoring" (the way you would plan to), but only for fun, and we wouldn't get overly worried if we couldn't find the heartbeat.
It turns out that we were never actually unable to find the heartbeat (sometimes it would take a lot of searching, and the machine wasn't really very pleasant sounding). Though we've pretty much stopped using the thing cause it got old after a while. I would say if a miscarriage is really a big concern, you should get the monitor, keeping in mind that not hearing a heartbeat doesn't mean that its not there.