I'm a journalist and media personality. I don't have GoPro, but many of my colleagues do. I have been looking at buying one. Most of my really professional work is in a real TV studio. A lot of my personal blogging and vlogging is done with my Samsung Galaxy tablet and phone. This is not what you are asking, but, I will assure you that my videos have been shown on TV news networks and the quality was great. I can provide you with some tips on how to kit a phone out, for less than $200, to make it a great video option.
Back to GoPro, Good investment, but, in my opinion, not very necessary for your project. If you plan to do more than this documentary, then it's a good idea. Frankly, I have two $1800+ digital SLR cameras and one very high end video camera that are collecting dust. This is my 4th Samsung. It fits in my pocket, a party purse, uses wi-fi and the videos are easily transferred to Vimeo, YouTube and a host of video editing software that is included in the Samsung purchase or free on Google Play.
I can also tell you about the accessories I use and where to get them cheaply.
I did several videos of my Granny before her death at age 91. I cherish this footage. It's been 7 years since her death. I'm SO grateful I decided to start that project when she was 84. I wouldn't steer you wrong with a memory so precious.
I'm here to help! I tutor on media subjects. I can help! Lucinda