My main criteria is that it has to be unique - not something identical or very similar to what I already have in my collection. And then it has to be in decent physical condition. I don't care if it runs or not, and unless it's really rare, don't need to worry about needles, bobbins or cases. Cost is the last thing I consider, though I'll always pick up a FW at a cheap price if I find one as they are really easy to rehome.