My personal opinion I disagree.
It only has a 250 litre tank - the reason why he is selling it as it's too small for a days work. 250 litres would probably last the morning, then he would have to go home to refill - not ideal. Plus side is that a small car could pull it.
It consists of a trailer £300 new, a 250 litre tank £140, a shurflo pump/strainer/Varistream kit £160, a battery £70.00, a bit of hose and a sealed electrical enclosure. No r/o, no di, no hose reel, no pole.
If this was the route I was going I would look for a good second hand box trailer,
http://www.ebay.co.uk/itm/Box-Trailer-7x-4-Carboot-Disco-Car-Trailer-/170949082890?pt=UK_Trailers_Transporters_Parts&hash=item27cd5bff0aFit a 400 to 500 litre tank in it, fit the pump, battery and Varistream inside in the dry. Yes, it would cost more initially, but will last a lot longer in the end as you won't grow out of it so quickly. Being 7' long, the trailer would be long enough to store poles in it as well. It would also look more professional and would have decent sides and back for sign writing. He would need a bigger car to tow it though.
In the winter it would be easier to keep warm.
Downside with trailers - easy to steal.
I would personally look for a 750kg braked box trailer. It might even pay him to convert this trailer to a braked axle.