In my opinion if you buy a big old heavy boat cheap
it will not be that once you add all of the upgrades and repairs
you will have to put in it. If it needs a more powerful engine
, bigger sails
replaced, bottom work etc, etc., etc, you should be ready to pay and put in all of the time for all of that as well. Then see if it is still a bargain or not.
Some boats are worth putting in the time effort and money
and will sell for much more if you put forth all of the effort. Others, because of their design or sailing ability may never be worth the efforts. Best thing you can do is educate yourself....like you are doing, before you make any down payments.