Top soil

At the minute I'm in the process of making a raised lawn roughly 4mx3m and 600mm high, will be quite a few tonnes of top soil, my questions are do I fill only with top soil or would you also use gravel, and how do I go about getting a good quality top soil and also without weeds, is this possible?

 

That works out at 7.2 cubic meters, which is about 12 tonnes of soil. Generally the better quality the top soil the less weed seed it has and the more expensive it is. Can be well over £100 a cubic meter and delivery can be expensive.