Yes, semolina gives the best texture to the base. It is not the same as cornflour, no. I get it at checkers. I'm pretty sure you can get it anywhere

To be honest, I wouldn't put the green herbs in the base, but rather the tomato base. I do have a tomato base recipe which I will post soon, but just some canned chopped tomato with purée,salt, pepper and your basil and oreganum is pretty tasty