My lemon tree was planted in a mixture of sand (to aid drainage) and bog standard potting soil, don't know if that is "correct" but it seemed happy enough and produced fruit. I think the most important thing is potting it in a medium that will prevent consistent dampness, heat and light in summer, and lots of food, and not to forget that the flowers need pollination either by bees or you! Now you've got me wanting to buy anothe lemon tree, my husband is not going to be happy, the first one cost us a fortune! Perhaps I'll try Aqui's approach and grow from seed, hopefully I'll get some seeds in my lemons!
Forgot to say, the pot I used was roughly 35cm diameter and 35cm height.