This is a trailer I built myself from an old caravan chassis, then I needed to imporove it when I went wfp. This trailer is now fully alarmed.
The main things you need to look for in a trailer are:-
It must be able to take the weight you wish to carry, this includes a full tank of water, hose reel, battery, thats just 3 of the heavey items we put into trailers, then you have to calculate what else will go into it, ie brushes poles, signs etc.
once you know what size trailer you want, then I would highly recommend a trailer with brakes, and breakaway cable.
I fitted rear legs to mine, this helps when unhitched especially if its unbalanced at the time.
keep an eye on the balance and the way you load it, all this affects its towing ability, unbalanced correctly and you will get what is called snaking when you are towing, if this does happen just release the throttle and let the car/van slow down on its own, don't apply the brakes.
Don't forget when the tank begins to empty this also affects the balance.
2 pictures for you from the original trailer to the new improved one.
JM123 is the best guy to chat with, I think he builds trailers to sell.