What you should stay away from depends on your skill level, ability to learn, and what sort of water you do want to paddle in.
The poofy Pink Mega X-ray Kayak I was wanting to buy-- not because of its color, was cheep for that type of Kayak, but it would not be a kayak for long flat water paddles, as it is mainly suited to the surf. If I wanted to paddle rapids, something I did when younger then some 3 Meter "Dagger" sit in Kayaks would be very suitable, and I could paddler these on flat water as well.
As far as price, I have 2 Fiberglass sit in Kayaks that need repairs for only $25 each, both have been patched before--and I purchased a kayak for only $20- at salvos once. [paddle, etc., would be extra]. I am not recommending these, but give them as examples of how cheap a Kayak may be, if you are in the right spot at the right time. One of these appears to be an old Down River racer, and the other I purchased for $60-- in a clearing sale. It is a 4 meter long Letman Slipper Slalom Kayak, great for white water touring, but needs work. I am about to fix and paint it, but price would increase then, as I want to sell all my Kayaks to get a plastic surf Kayak-- no more repairs, or if a repair because the plastic goes brittle when it is 10 years or so old, by plastic welding.
I am putting these into the Tender Center to sell them.
I did surf a Canoe Polo Kayak 3-4 days a week, for nine months, a Kayak that cost me only $49-- from a Kayak shop-- it needed a repair, which I did when I purchased it. I am planning on doing a repair andoffering it to an interested man for $195 with a paddle-- it was the one I was going to keep as a spare, but this guy wants this 2.75 Meter Kayak, and I want the money to get that surf Kayak-- similar Mega X-ray on Gumtree Mega X Ray Surf Kayak 3 Metre | Kayaks & Paddle | Gumtree Australia Noosa Area - Noosaville but the ad is wrong, they are 2.25 Meter, not 3 Meters. But the poofy Pink one that I was chasing was only $350-- a better price, but in South Australia, hence transport/distance problems. These are $1000 New, which I cannot afford.
I also have a early Lettman olymp Slalom Kayak that I hope to get $195 for with a coat of paint paddle, spray skirt and helmet.
Also I have an old Chez C2-- two man canoe like a Kayak, but normally paddled differently, like a Canadian canoe. It needs repairs modifications and I would let it go for about $200 with crap paddles. And I do not recommend this one for an inexperienced paddler.
Do note, I am not saying you guys should buy any of my Kayaks, I am giving examples of how cheap old second hand white water Kayaks can be.
And I no longer paddle rapids but used the slalom single man Kayaks in the ocean--near to shore, and have surfed waves with them.
I should tell you why we should not surf these. The 4 meter long slalom Kayaks turn sideways to the wave, so while you are bracing with the paddle, in an effort not to be tipped over, your Kayak is mowing down little kiddies with their floaties, as you have no control over it, and they are too young to get out of your way.
The Canoe Polo Kayak at 2.75 Meters was controllable in that I could miss people who got in my way.
I used both of the slalom Lettman Kayaks, and the Canoe Polo Kayak to explore numerous coves in the Tweed River when it was too windy choppy to go out, and paddled them a couple of Kilometers along the coast, etc.

