Cashmere is delightful, soft and not itchy at all. That said, there's a huge range in quality and price.
The best cashmere is Scottish, but it also costs an arm and a leg. Brands like Johnston's, Pringle, Kinross, etc. It's really luxurious, soft, and more durable. Cheaper cashmere these days is from China usually (although actually I think most of the fiber comes from China and Mongolia anyway, but the Scottish cashmere in still spun and knit in Scotland, and I think they may get the higher quality fiber from China). I have a Kinross sweater that is old and still going strong, and it feels softer, warmer, and more luxurious than my $60 Uniqlo or even my $250 Autumn Cashmere.
In general, the more plies the better and more expensive it will be. It will also be warmer and thicker. Plies refers to the number of individual threads that are twisted together to form the yarn. I find that the cheaper ones are generally 2-ply, which is quite weak and prone to pilling. 4-ply is decent, and 8-ply is really nice.
Regarding washing, I am a big proponent of NEVER dry cleaning cashmere (unless it's a woven, like a coat or something). Or merino for that matter. Basically, dry cleaning chemicals weaken wool fibers, and short staple fibers like cashmere and merino (essentially the softer ones) are the weakest to start with, plus I just don't like dry cleaning chemicals in general. You do better hand washing (or, if you have a nice washing machine, use the hand wash cycle) without agitation, and then blocking flat to dry. Definitely wash them regularly (especially right before storage) as moths prefer dirty wool.
If you want to go for a blend, I would recommend silk-cashmere. In fact I think it's preferable to 100% cashmere if you want something very fine gauge and thin, as the silk is stronger. It has a nice sheen too. I'd go for at least 20% cashmere to get the softness; 50% is really nice.
