A few thoughts: used, older, full-length (16-18ft) fiberglass kayaks regularly seem to come up on Craigslist for sale in the $500-$800 range–and sell within a few hours. I purchased mine for $650 a few months ago, and have seen two more similar deals since then. You have to set up an app to notify you when a ‘hit’ comes up, and be ready to jump on it right away.
Sea Kayaking is an “aging” sport, lots of people retiring from it and selling off their boats, often with lots of ‘kit’ (paddles, pfd’s, etc.) included.
Used folding boats seem to sell in the $2000 plus range. If you don’t absolutely need a folder for storage reasons or because you need to strap it to your back to hike up to mountain lakes, consider a hardshell. If it looks like a ‘beater’ it could probably live on foam pads on top of your van without getting stolen.