For the first few months i used my car all be it a small car Renault Clio Sport, it was a nightmare, although getting ladders on and off was ok as it was low space was an issue.
As you say 2 buckets and a box of scrim doesnt take alot of space up but it is soooo much easier to have a van plus im not too bothered if i drop water or dirt/rubbish.
I have a Citroen Berlingo and i would say its just about the right size what i have in mine is
Box containg 2x 10ltr water containers
Box of dry scrim
box of clean cloths
box for dirty cloths
2x extendable poles
stand off
Box of "bits"
Box for site boots hard hat water proofs
and occasionally fishing gear for a sneaky after work session
Also with a van its always advertising i have just had signs put on it and over the course of a week gained 2 gutter jobs and 2 new monthly window cleans so already the signs have paid for them selves in 1 week!!!!
Just about right for me, no way go back to a car
Here we go a picture for ya