Hi
I started with one of Peter Fogwill's 375 liter trailers towed by a little Suzuki 1.3 Carry that I owned before I started WFP. (According to the local trailer suppliers and fitters, this arrangement wasn't man enough to carry the weight and had no end of problems with wheel bearings - in my experience with this trailer I would recommend anyone considering it not to.)
My experiences:
As the trailer was 'open' - 3 x 125l tanks secured to a frame - winter time was a real problem as everything just froze up - it was parked on the drive at night. The pump was in a pump box which could be removed and stored in the house on cold nights, but that didn't help with the supply and delivery hoses frozen. Because everything was so exposed, travelling to your first customer usually meant everything was frozen, even if it wasn't when I started off. Ideally, the trailer needed to be stored in a garage out of the elements.
If you are considering a trailer then an fully enclosed trailer with a lid on that has room for a frost heater is a very important consideration.
Secondly - buy a proper heavy duty, sprung, galvanised unit with decent tyres that will last. A braked 750kg would be ideal, but are much more expensive than a non braked equivalent. Twin axle trailers will tow better and be more stable but would be almost impossible for you to manouver by yourself.
Also ensure that the tow vehicle conforms to towing legislation for the weight. When my trailer was full it exceeded the max unbraked trailer weight that my van could tow, so had to carry an extra 80l of water in the van as 'ballast.' Remember that when buying a trailer unit, you still need to carry hose reels and poles as well as your usual gear. So you need to find a way of doing that.
A trailer can sometimes be difficult to reverse - the shorter the base length, the more difficult to reverse. Always seemed to get into trouble in cul-de-sacs if there were too many cars parked around. If you need to uncouple an unbraked trailer on a hill with the trailer full of water, there is a pretty good chance it will end in disaster.
I looked around for a 750kg braked van trailer to replace this trailer, but they were more expensive second hand than a Fiat Ducato van, so opted for the equivalent of the later, a Ciroen Relay (van mount), and to be honest, this is the way I would advise you to go.
If the 2 of you are going to work together WFP then you will need a fairly big tank. My son and I work together off my van with a 650 l tank sometimes (2 man setup), and we can polish off 650l in a day easily.You might consider a back-pack (son can work with it all day but I can't) or a trolley which will allow one of you to work further away from the van on your own. 2 people working efficently WFP of one van is more difficult to achieve than Trad.
I see there was a 2 man setup trailer on Ebay recently for £2000 with pumps and reels, but the tank was only 250l. They had a Merlin RO in the trailer as well - the theory probably was that they could process water from customer's taps, but that isn't always ideal, especially if customers are on a water meter - it would open a whole can of worms.
If you go the trailer route, I would suggest that the leisure battery in the trailer to run the pumps be linked to the tow vehicles charging circuit via a split charge relay, so at least you can run the tow vehicle's engine to give a flat battery a boost if you get caught out - happened on a couple of occasions to me. You could also couple another cable through to the trailer with a change over switch that would allow you to draw power from either the trailer of tow vehicle battery.
These would be the same wiring connections that caravaners use
Spruce