Is this warm enough as a winter base layer?

Yes. Fine-gauge cashmere insulates well for its weight, and the roll neck covers the throat. It works under a coat or suit in normal winter cold, but it is a layer, not an outer garment.

How does it compare with a wool turtleneck?

Cashmere is softer and lighter than merino or lambswool at similar warmth, so it sits closer to the skin without itch. It is also less durable and needs gentler care.

Will it fit under a suit jacket?

The fine knit is thin by design, which is the point of a base layer. It sits under tailoring better than a chunky cable knit, but a close-cut jacket may still show the knit's outline.

How should I wash and store it?

Hand wash in cool water with a cashmere shampoo or dry clean, then dry flat. Fold it rather than hanging, and store with cedar or lavender against moths.

Fine-gauge cashmere commonly pills at first, especially under the arms and where a bag strap rubs. A cashmere comb removes the pills, and pilling usually settles after a few wears.

Is it worth it over a cheaper cashmere turtleneck?

The difference is the yarn and the Italian hand-finishing, which show in softness and shape retention. If you want a thin, long-lasting layer and will care for it, it earns its place; if you want something to wear hard, a cheaper wool knit is more practical.