We've fallen in love with a 35' boat from 1982.
Never fall in love until she's at your slip. It affects your judgement. Still, I'm glad you have a strong prospect.
1. Does it make a difference on the life expectancy of an engine if it's a fresh water vs salt water boat?
Not necessarily, it depends entirely on how she was used and maintained. Fresh water environments theoretically are less corrosive on the boat in general, but there are poorly maintained fresh water boats and expertly maintained salt water boats that debunk the generalization.
2. How long should be expect this engine to last?
The range is too dependent on the last answer. If this motor has never had any major surgery to refresh wear parts, it will need it before too long. That's just age appropriate.
3. Would it be worth it for us to do an engine survey before purchase, and is this common?
Yes and Yes.
4. Would we do the survey before or after the marine survey/sea trial?
If this is your primary concern, I would do it first. If your purchase price is dependent on a serviceable engine, you would avoid the cost of the survey if a problem is detected that you can't reconcile with the current owner.
5. How much do single engine surveys typically cost? We are in the Chesapeake Bay area near the coast.
Less than the hull survey, most likely. It will depend on what they do. Is it just a static test at the slip, testing compression and listening. Will you have an oil analysis done. How about checking valve timing, etc. Find a local reference. No one can give a guarantee here.
6. Should we just say hell no to a boat with a 35 year old engine? The boat is listed at 45K but the owner is ready to negotiate.
If she starts easily, runs smoothly and doesn't have notable exhaust smoke, it could be just fine for now. However, also understand that it will need to be overhauled or replaced at some point. That's why you're not paying new prices.
Also: this is my first post! Hello Sailnet! I'm super excited to find this website