Gaz the tips I can give you is make sure its a braked one, mine is only 6'x4' with brakes and has a total weight of a ton, but with a 250ltr tank in it, I just have enough room for my hose and reel, pump box, and battery, so if you plan to carry anything else in it, go for the biggest you can.
I also welded support legs on the back of mine, so if I was loading say a fridge on it, I would drop the legs to stop it tilting up. and they serve me well for when I am reversing it, I can see the back of the trailer by the legs in there raised position.
A Jockey wheel is a must if you want to move it easily with a full tank.
For Revesing they should have the self release brakes, just like your caravan does.