Never tried kayaking, but now, I want to. Help me buy the right one...

Hi all.
I just attended a 4th of July Freedom Swim where I live and it was fantastic! It is a "swim" across the Peace River--which flows into the gulf of Mexico--where everyone takes boats, swimming pool toys, things to float on or whatever, to get across.

A few people had kayaks and they looked they were having fun while still getting exercise. My husband and I looked at each other, and said, "Why on earth don't we get one or two of those?" We live quite close to the water and already have a truck to haul them in/on.

But, we've never been kayaking! I have a feeling it is much harder than it looks. We are gonna rent some to see what we think. In the meantime, I'd like your input about what to look for when purchasing used ones, how to tell whether we want one where our legs are "inside" versus on top of the kayak, two-person versus one-person, etc., cost, etc. Thanks for your advice on this one.