The biggest problem with "free" boats is not the obvious, cosmetic stuff.... no... it's the fact that they are usually missing major functional parts. At some time the previous owners realize the functional pieces from this non used boat were worth more than the boat itself... and started parting it out to others who were in the same predicament you may find yourself in....
If you can verify that all important gear is present then I would say.. go for it! You'll have a lot of fun cleaning and sanitizing, sanding and painting, reworking the wiring and plumbing, plus, becoming a regular at local and online chandleries. (no such thing as a free boat)
If you can't, or don't understand what I'm even talking about, then just be ready to spend an unreasonable amount of time and money trying to find fundamental replacements. I'm talking about: Mast, Sails, motor, trailer, centerboard/keel, rudder/steering system.... and hull integrity, including hatchboards, portlights(windows), hull/deck joint, chainplates and standing rigging (the wires).
It's a lot of stuff... anyone of those pieces are missing, add at least a thousand dollars to the cost of your FREE boat.
My recommendation is to go through the boat again, after reading a few books on purchasing used boats, and talking to someone whose opinion you value... and make sure you can locate all these important parts... if they're missing, ask the owner what he knows about them.... If really a mess, he should PAY YOU to remove it.... 'cause it's junk.