Hi Davaw,
It is a little difficult to say with any certainty how good this is as it is only as good as its perishable parts.
I presume it has come OUT of a van as it has clamps for fitting.
It has a 40/40 RO which in itself is good unless the membrane needs changing. It has softeners and DI which are Ionics, personally I would throw these and buy a new DI vessel and resin to polish the water.
It has a pump which will be fine if it runs if you do buy check this with a 12v battery first.
A water meter is handy but not essential.
As for the other bits of 'missing plumbing' you wouldn't need to buy them from Ionics as you could get them from any supplier.
You are essentially looking at a tank (400l?), pump, and RO casing. With a few bits of pipework. The RO may be OK but presume it needs a new membrane. Take both these into account when bidding. You can buy these from most suppliers.
You would need to add a DI vessel for polishing and if you are not putting it in a van another tank for your van, unless you are using 25L containers.
As for what it is worth?? £300-400?? considering what you get and what you would need to replace.