I just bout a used Chesapeake 18 in great shape. I have to keep it outside and advice on how to keep it in good shape
for outside storage consider storing upside down to ensure water won't get in the cockpit and sit. make sure you put it away clean
ensure supports for it are close to bulkheads to avoid warping/compressing the bottom. wide supports to spread the load are also a good practice.
key thing is to keep the sun off of it and to let it breath....don't want to hold water against the hull with a saturated/wet/dirty tarp. ensure it is secured against windstorms, mud and debris...and critters :)
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I agree that storing the boat upside down is a good stratgegy. I stored mine upright one season and the cockpit covers allowed enough water in to nearly fill the boat. I was amazed that the Harken Hoist didn't pull out of the deck joists! The water sat in the cockpit long enough that I had to dry it thoroughly and do major repairs to put the Tred Avon tandem kayak back into service...lesson learned. Water sitting long term will find its way through the best sealing job and start the rot process.