After the first ride, any new boat becomes used and worth substantially less, so you might want to drop that from the equation. 'nuf said.
When the 20 Seacraft was in production one of the best motors for it was a 2 stroke 115 (~330 lbs I think) - it was pretty awesome; efficient and handled really well. If I recall correctly that was crankshaft horsepower and not propshaft horsepower so a modern 115 is equivalent to a good bit more and is a whole lot more efficient. I think that Yammie 115 is about 380 lbs and the Armstrong brackets are Al (not being made of stainless is good there for a whole bunch of reasons starting with mass). The HP seems right to me and the weight aft only slightly high.
As far as being an all-around hull (inland and ocean), you would be hard pressed to find a better one.
I think that year was all composite, or close to it and you should verify with others, but the chronic rot (deck & transom) issues should not be significant.
One advantage of an older boat is that it has been 'seasoned' to the point that you should be able to see any problems.
Have fun with your decision.
