I paddled whitewater for a while and gave it up. If you buy a boat get a stable mid range boat with a flat bottom and volume. You buy somethi g small, dont know what you are doi g and get into a decent hole and you could easily stare death in the face.
Never paddle alone.
My best advise would be find someone patient and trustworthy who alreay paddles. Get them to teach river safety, reading a river, etc. Borrow their boat and get in a river with them. If you like it you will begin to understand differences in boats and which one would suit your needs.
Be safe... I have lost friends to the river.