Hello Ben,
It all depends on what future clientele you aim at.. The occonnor heads with ronford legs is a sure thing, I would also consider at a long zoom. (alura 45-250?)
Also try and find some old super speeds so yo can also serve a smaller budget (but also some DP's like those)
For documentary films the F 55 might be popular with some lighter zooms and smaller tripods (miller or Sachtler 100 mm bowl)
Maybe a RED ? (with a ronin)
And accesories easyrig, filters, monitors extra batteries stuff like that...
I think service and helping to make a production possible is whats being appreciated most in rental companies...
anyway good luck