The slips take a WHILE to develop, like 6-8 weeks I'd say. And as long as you use a whole potato it'll keep producing for many months, so no reason not to get one in
water 3-4 months ahead of time as long as you've got a sunny window or a grow light. It's really quite decorative.
The white fleshed ones taste much nicer and are more versatile, unless you like funky carrot flavor that is. You can get cool climate varieties in orange and white, but all the purple ones seem to be tropical, curiously. Wouldn't go with the store ones if you can possibly help it.