I was honored this year to be acknowledged in Blain Brown's book (Cinematography: Theory and practice | Third Edition | 2017 ) for my contribution, help & support.
If you ever want to read this book then make sure to get your hands on the current edition of this book.

are you working on H.264 ? if yes, then try to ingest Prores 422 HQ and work on it..

I know the problem you are facing.
You shouldn't edit on H.264, Resolve hates this codec. Most of the editing softwares hate this codec.
Try to edit on Prores 422 HQ or anything else but never on H.264, Too much compression!

short answer: It depends on the type of shootings and projects you want to work on.
In general, most of professional cinematographers own some personal equipments..
like (light meter, color meter, grip bags ect)..
If you want to be 1st AC its good to buy a professional wireless focus system..
for director of photography you dont need to own everything..its good to invest in cinema lenses because digital cinema cameras are always in change.
However, its good to invest in Light, you keep it personal..
other than that it depends on what kind of projects the clients want you to work on..
most of Steadycam operators own their own steadicam because many producers when they look to hire steadicam they search for someone who already own it (easier to contact one person than two, besides its cheaper to hire someone with his own equipments than to hire him and rent by your own, Think of Assurance responsbility that you will take off your shoulder as producer or cinematographer when you hire someone with his equipments)..
So In general, its not bad to have your own essential equipments (light meter, color meter, slider, essential light equipments, cinema lenses)..
but its not recommended to buy expensive cameras as they are always in change except if your budget allows you getting Alexa..
(for Red and Black magic or anything else its cheaper to rent than to buy, and technically speaking renting goes on the shoulder of producer and the client should pay so it should not be a problem for the cinematgrapher anyway)..
Except if you are thinking of not more than Dslr shooting projects then its good to own the camera too if your work always requires that...

thanks Cédric excellent book La Dramaturgie..
I found this link that compares the difference between Yves Lavandier and John Truby method of writing..
I think both books are on the edge of the top books on this subject.
http://analyse-scenarios.over-blog.com/article-john-truby-contre-yves-la...

its an excellents brief for crash course..
for further more, the following book is the script writing Bible fore everyone seeking writing a professional screenplay for film.
The Anatomy of Story / JOHN TRUBY
one of the best scriptwriting books outhere.