I wouldn't necessarily agree with older ewes coming with problems, they should have been kept in their flock until that point as they have been successful sheep, not those with problems. I don't know about in the area you live in, I would imagine it is the same for some farms, but because our farm is pretty rough going, we only lamb our sheep four times, then sell them as drafts. They are probably the easiest thing we sell, there is good demand for those warranted sound in mouth (we would consider this to be at least six teeth left including the front two) and udder. Some buyers lamb them once more, some twice, some until they die. You'd probably need to lamb your first lot twice until your first lambs you bred were ready to lamb themselves. Up here you would be able to buy good serviceable draft ewes, without being anything special, for £55 - £60. Is going to be more for a pure breed is they are rarer, though I don't know anything about Shropshires specifically.