My main concern is trying to determine what shape the hull is in without actually pulling it out of the water.
Sorry it can not effectively be done in water..
Consider that a survey is USUALLY FREE!!! I have yet to actually pay for a survey and I do work with a friend who is a surveyor so I am surely competent to do my own. I ALWAYS use the survey to knock the price down and it always pays for its self.
Problem is a DIY survey will get you virtually nowhere in negotiations. Do you own a moisture meter and know how to use it? it will also get you nowhere with an insurance company.
A good surveyor may get you that boat for 4k as opposed to 8k. You may get it for 7.8 k after a DIY survey for which you don't have the credentials SAMS or NAMS to properly negotiate down the repairs needed.
Without a moisture meter alone, and the "know how" to use it, a DIY, even on an 8K boat, will likely cost you a lot more than the survey.
Of course you apparently already seem to know it all and have not taken advice from those of who have "been there and done that" for the better part of 30 or 40 years as many on this forum have.
I've seen 12k boats turn into 40k nightmare/money pits because a buyer thought he could save $500.00 clams on a survey and do a Don Casey DIY survey..... It's your dime and remember a good survey costs you nothing and it's all but guaranteed to cost you nothing on a 1979!
Heck I bought a BRAND NEW boat in 2005 and found over 2k in issues that needed to be fixed before delivery with a survey. Mostly gelcoat and void issues..
A deck job, for wet deck cor, on say 20% of the deck, not out of the ordinary for a 1979, would run between 7-12k depending on the yard. A 7-12k deck core repair is a structural issue and essentially TOTALS an 8k boat. You may get this thing for free with a $500.00 survey and it would not be the first time this situation has happened...!