Welcome to GoY!

Try well rotted manure and topsoil, compost (from garden centres) only dries ou and doesn't actually improve the fertility much, after all they only conatin enough food for a year. Compost inproves the look of the soil only. Home grown compost on the other hand is ideal.

Its difficult to say how much. I normally put down 1 x 60lt/70lt bags for every 1-1/2square metres, dig it in well and as deep as you can into the topsoil. The supplier maybe able to tell you a quantity per meter. This may seem alot, but it will encourage wroms to take it deeper. Rotted manure will add food for the plants and help retain the moisture.
You can buy bulk bags of soil improver/conditioner which i tend to use on big jobs especially if the soil was previous lawn or if it had just been left. It comes in bulk bags which tends to work out slightly cheaper than if brought form the garden centre. Go direct to the supplier if you are planning to buy a lot esp. if you want compost sized bags rather than bulk.

Digging it in will also bring any roots that have been missed, up to the surface as there will inevitably be some whenever you dig. Leave the soil for a month or two to settle and also to allow any weeds to re grow (esp. docks and thistles) then dig these out our treat with glyphosate (such as Roundup) as if you didn't get them out completely and some snapped off, then spraying will get the weedkiller straight to the root, killing the weed.