pasture grass....any advice?

we are looking into buying 18 acres of land, but I want to get some more information about the pasture acreage first.  I have all the soil info, and plan on inquiring on what type of seed to sow that will work best with that type of soil...but can anyone give me a roundabout idea of what it might cost to eliminate what's there now (kind of a combo of regular grass and something that looks like wheat, but is probably more like a weed) prepare the pasture areas, and seed with the proper grazing grass for horses?  I imagine we will make 4 acres of it pasture for now, as we also have to fence it in.