I use an estate car for most jobs now. £100 to fill up the van, £40 to fill up the car, same mileage for full tank...........you do the sums. You don't need a van full of everything just to fit a loo! When fuel was cheap we thought nothing about whizzing about in the van all the time but it can now be up to £100 difference in a weeks work!
For larger jobs I often use the van to begin with and once I am started off I use the car. My electrician uses an estate car too and there is plenty of room for all his kit in there.
I have a number of tote boxes (the heavy plastic ones with the folding lids), one for each type of job (eg oil boilers, general plumbing (with silicone, cordless drill and fasteners), Pipe fittings etc, fasterners, waste pipe fittings and traps, and so on, plus one tool box for plumbing tools and another for oil boiler tools. For example if I am fitting a basin I grab the plumbing toolbox, the plumbing tote box and the wastepipe tote box in the back of the estate - therefore everything I am likely to need all together.
As long as your car is clean and tidy. If you use the car for leisure then seat covers are a good idea.