Hi, Lou! Welcome to the funny farm. You'll get to realise that my posts are long and boring and that I am a windbag! Hopefully you'll find something of use in them - "hid treasures" and all that!
Yes you can certainly do what you are suggesting.
In the long run the van option will make you more money. (IME)
My experience as a family man.
When I was trad. I used various cars the last one being a Volvo 240 estate and then a Honda Accord Estate. The Volvo was a bit bigger and I went wfp and had a trolley with 8 barrels (200L) in the back folded down. The Honda would hold 6 but I put 1 or 2 in the front footwell.
Now this is the thing ... they would last me all day when I started but as I got used to the system and custies layout I sped up and found that by about 3pm I was running out of water. So I would start doing downstairs trad at about 2pm to make the water last.
Then I put a 175L tank in the back and with a few barrels I could carry 250L and so that meant I could work quicker (I now had a reel to the tank plus the barrels for the trolley).
This was very flexible as I could use the reel for speed and the trolley for awkward jobs.
A French lorry driver pranged the Honda and I bit the bullet and went for a van with a 400L tank and found that as I sped up again I was using that amount of water by 3pm on a busy day and now carry two extra 25L barrels for topping up. When my van changes this time next year I will have been wfp for 5 years and I think I will go for 650L/800L of capacity as I am thinking about occasional help on board and a less cramped work area.
We also have a big Pug 406 diesel estate family car.
Of course with an mpv you could probably carry a 250L tank in the back and a couple of barrels in the footwell but it will get knocked about and a pain when you have to take everything out for the family. It will also smell like a window cleaners car (damp and musty) after you have those inevitable leaks or bring dogs poo in on the trolley wheels or store the rags you've used to wipe the hose and said wheels!
Buy the van if you can - but use the car to start if you need to.