Technically Documentary filmmaking has become a lot easier. You could even shoot a documentary with a smartphone. But there are a lot of other things to consider when going for the long form.
I made this video for the International Journalists Network and tried to break it down to the eight most important things that I stick to when doing a documentary. You can find a more detailed description about this topic at https://ijnet.org
They provide many more great resources on their website.
And here's the promised list of my eight documentaries that will really inspire you as a filmmaker.
This mini series by sdun.net is a free introduction into movie making and video editing with your smartphone. Tutorials on Film Making Basics and How to Videos on Time Lapse, Super Slow Motion. And there is more to come.