First, if you don't mind a coat from a thrift shop, there are dozens of overcoats and topcoats you can find from almost any maker at Goodwill and other shops in most areas. Sometimes, if you are lucky, you may even find brand new or NOS coats, often with the tags still on them. The prices I have paid range from $10 to $25, for 100% pure wool, 100% cashmere, or wool/cashmere blends. I have not bought an HF overcoat yet, but I have one from Hart, Schaffner and Marx, almost new. It's excellent and a great fit. These low prices give you greater flexibility to experiment a bit and try out different cuts and styles. Also, if you really like a particular garment or house style, then you could spend more money and buy a new one in that style.
Second, I have read (here in AAAC, if I recollect correctly) that cashmere is softer and finer, and therefore somewhat less durable over time than pure wool. Since overcoats can get fairly heavy or rough wear, this might be a point worth considering. But if your 30-year-old herringbone coat was cashmere, and wore well, then you could ignore this issue. I'm sure there are variations and grades in the quality of cashmere, like any other material, so this factor can influence your decision.