I bought a "Noval Portaflot" trailer secondhand. (Think "Erde" like the Halford's ones - but at the top end size wise) It was about £120 quid I think with a canvas cover.
In old money it is about 5ft x 3ft and has a payload of just over 410kg and a total weight limit of 499kg so just under the weight limit where it needs its own braking system.
Changed the bearings which are about £25 a set, bought a spare wheel and put a 250L tank in it, built up the sides with ply, put an immerson in the tank, a reel sat alongside but could sit on the top when parked.
Dropped the battery (70ah) in at the start of the day, switched on the shurflo pump and ready for action.
NOTE: This was an add on to my Doblo system when Dan the Man started. The Doblo with 400L and the trailer with 250 gave us enough for the day. 6 months later when work had built up I got a bigger van and the trailer had its tank removed and became an all purpose vehicle which I use for carrying a generator and guttervac - or more accurately Dan does.