I dont know how coarse it is, i've got some standard gravel that you get from pet shop. Nothing small, nothing huge. Its just normal size gravel?

Whats the easiest way to get CO2 into the tank?

With my size tank (90cm x 30cm x 45cm), how many KG should i get of the sandy substrate? And how deep should it be? 2 inch?

Is the sandy substrate the right way forwards?

A 10-15lb bag of playsand should suffice. Build it up to a layer of no more than 1-1.5 inches, if the substrate is too deep it will compact and as a result small pockets of hydrogen sulphide producing anaerobic bacteria. It's advisable to stir the substrate with your fingers once a week or so to ensure enough oxygen circulates through the layer.

Not only does sand look far more natural than gravel, it's also less of a hindrance to plant root development.

So this will do?

B&Q Play Sand Natural, 0000003377277

That'll do.