Be very careful tonio horse boxes are very heavy & with smaller wheels they over heat the tyers and burst, I have 12" wheels on my trailer with 8ply van tyers a good compromise me thinks.
Even a light-weight tow-a-van doubles the fuel consumption. Its not the weight - that only really slows your acceleration - its the frontal area of the bloody things that sucks the juice out of your tank. We've got a tow-a-van, a horse trailer and a 3-bike trailer and I've towed them all with my car and monitored the fuel consumption (well it has an onboard puter so its easy)
Anyway - I would encourage anyone who is thinking about getting a trailer with a roof to go ahead - they are wonderful when it rains :P
(wouldnt want to p*ss about changing the wheels tho)