I think the quality of the topsoil is also quite important, if it has too many weeds etc in it then you have to start killing the weeds either with chemicals or large plastic sheets.We had an offer...tax free from a local geezer doing other local work for 300 euro for the lot. Sounds so good, but I'ld hedge that he'd do a crap job, not put down 12m3 and wouldn't come back to sort it when it all compacts to a roller coaster in a few winters from now..
By the way I remembered we had half a truck of topsoil delivered for our garden, which turned out to be way too much and we told our neighbours to please take what they needed from our pile. We ordered it from a company in the yellow pages, but it was not cheap.
Our Chrysler Voyager has a 1600kg towing capacity (our trailer has brakes) and it is a small engined version and our Jaguar has 1570kg towing capacity so I am a little surprised by the capacity of your car. My trailer weighs in at just over 400kg (and can carry a 1.2 tonnes)