What about things like cranes/dollys etc. I want to try my hand at designing and making some of my own, sourcing out any welding when I need to. Would this be snubbed by some directors, even if they were genuinely up to par with good quality branded versions?
Yes.
Remember, good dollys are so expensive you can't rent them (Chapman; etc.) and low end jibs $1000/day. The more you invest in cheap gear the more you water yourself down. And I thought you wanted to do documentaries?
Unless you're just doing it for fun or as a self-contained production company, in which case results are what matters... to some extent.
For a documentary, C300 and sound devices 302. Any decent lenses.
I find the Zeiss CPs fairly meh, but they rent well and are well-respected. I just find the look boring, they're totally decent lenses.