Buffy, Grassland management is my speciality subject! Sheep do not need you to fertilise the grass, they are the perfect grass managers which is why they are used so much in conservation grazing where short species rich swards are either already there or are to be encouraged.
If land has been over grazed by horses sheep are the perfect antidote as they will eat down the sour areas and the action of their feet will act just like a chain harrow and roller!.....at the same time they are mobile fertiliser producers, returning to the land the waste produce of what went in!
If you overstock, which I am afraid too many people do then and only then you will have problems. Keep a max of 4 ewes to the acre plus their lambs in summer and you will not go wrong. Cut nettles, which usually occur on poached horse ground and leave them to wilt in situ and the sheep will munch them too...v high in protein. Do not worry if grass gets a bit long in summer, it will be flat grazed by November.
Fence it very well and divide up into 3 or 4 paddocks/fields so you can rest and rotate....simple!