Is it listing with a broker? If so, you might be better off using the services of a buyer-broker. He/she can walk you through the entire process.
If you are strictly a DIYer, start with SailingDog's boat inspection tips
. Then if you still want to buy, make an educated offer contingent on survey and sea trial. Make sure YOU pick the surveyor. If they accept your offer you will be expected to put some good-will money into escrow. After the survey you will have the opportunity to haggle further based on findings of survey/sea trial. Agree on final price and pay the remaining balance.
Either way, make sure you ask the broker(buyer or seller) for comparables from Soldboats.com so you can get an idea what similar models have sold for recently.
This is a good site for researching Cape Dorys: Welcome to the Cape Dory Sailboat Owners Association's Home Page