Look up Fro Knows Photo/ Jared Polin. He has two guides all about video and editing for about £150 each I would solidly recommend. Or just go on his youtube channel where he as given so much away for free, I've learn a hell of a lot from him. He's a Nikon shooter too.
Other than that just trawl Youtube as there are thousands of tutorials and tips etc.
One thing I'd say is get lenses with (IS) Image Stabilization or VR (Nikon) OOS (Sony) as camera shake, unless used properly for effect is just messy.
Cinematography, pacing, location, colourgrade, it is a beast of its own but very exciting. Coming from a self-taught video based background into photography I find video easier but they share similar qualities, its all about making images.
In terms of manual focus, I've just upgraded from a 5D mk iii to a Sony A7rii using my Canon L glass and I can say the more expensive lenses are much better for focussing as you can feel what impact you are making more.
As always its just practice and knowing your gear, the first documentary I made: Val's Gym, which is exhibiting as part of Bloomberg New Contemporaries 2016 in Liverpool and London was done on a Canon 70D with the kit lens using the Dual Pixel AF autofocus throughout.
External monitors are handy if you really wanted to go for it. But the first thing I'd say is get a decent microphone as it gives the video a very professional feel even if the camera work isnt the best.