Good advice Gillie regarding leaks and rot in the decks and transom! Soft decks and transoms aren't that hard to spot if they're really bad.
The Seafari 20 is an incredibly capable 20' boat, the most versatile of all the 20' models. Compared to a center console, it has more weight forward so it's a little better balanced and therefore rides a little better IMHO. It's also relatively rare - they only made 741 total! 70% of the 20' hulls were center console models. I personally think it has the most beautiful well proportioned lines of any small boat, except maybe for the Seafari 25 and the REALLY classic and legendary SeaCraft 21 that started the whole works! I've had mine since 1975 and have yet to find another boat of that size I like better! Other than having the transom cutout filled in a couple of years ago, it's all original and rock solid. Of course I've been real fussy about sealing any holes in the deck & transom and I keep it covered. I've put a couple thousand miles on it on trips between Fl. and the Abaco's (~170 miles of open water), so it's seen plenty of rough water. Not many other 20' boats I'd want to make that trip in!
Regarding the Force motor, that was originally made by Chrysler. They were very light, simple motors, and I think these boats ride best with light motors. I believe Fr. Frank had one on his Seafari originally. Sounds like it has a rebuilt powerhead or short block which would be good IF it was done right . . some folks take a shortcut and only bore one cylinder if that's all that was damaged. If you do that instead of boring all cylinders and installing all oversize pistons, one cylinder will have more displacement than the others so it'll tend to run a little rough even if all the pistons are the same weight.
Regarding differences in years, from the boat's I've seen it looks like Potter changed from 4 f/g stringers (like those shown in the Boating Magazine article on the Seafari in the Literature section) to two wide stringers starting in 73, but no problems have ever been reported due to that as far as I know. Starting about 74 or 75, they raised the cockpit sole an inch or two
, installed a larger fuel tank
but the cockpit stepdown aft of cabin door got much smaller, just big enough to open door.
(The larger stepdown is a great place to put a 70 qt cooler in rough seas to keep it from sliding around!) They also raised the windshield a couple of inches with a spacer section at base of windshield and moved it forward about a foot, so there's a little more headroom in the cabin and more space for instruments behind the windshield
.
Read "The Amazing Mr. Moesly" article in the literature section to better appreciate the clever and sophisticated design and construction of these boats. A close look at the stringer and inner liner design in the Seafari magazine article will reveal why these hulls are so stiff but yet relatively light . . . the hull is built like a big I-beam, much like the wing of an airplane!
Good luck and keep us posted on what you find out! Denny