Hi
Just say i agree with most of what has been said. RO is to make high ppm water pure in the most cost effective manner. this does not have to cost a fortune. As has been said an ro, ibc tank and transfer pump can be bought for under 300. I use a merlin these are readily available.
You will recoup this money very quickly as resin is very expensive, it is also a hassle filling the vessel which you will quickly get fedup with the amount of times you will have to do it.
As regards a van mount i would say in most cases you are better off, but a friend of mine has terraced houses and finds wheeling the trolley down the entry is easier than pulling hoses round, especially with parking issues.
van mount also doesnt have to be proffesionally system, just by a tank and pump make sure you secure it well though.
one thing is you will have to change the van as the round builds, 250 will not last past the morning so you will need to go home for lunch and fill up. I deally you need 500 or 600 litres perhaps even 800-1000 but you need a transit or vivaro for that.
all of this has been said but just to confirm it
cheers
john