I think anything under 7 would be fine. Several areas are constricted by less draft (i.e. Rio Dulce entrance). This seems a good compromise over leeway vs access to anchorages. You didn't mention keel shape or retractable keels so I'll leave that subject.
2: Steel vs Plastic:
I don't see a problem with well-constructed plastic unless your passage includes high latitudes or significant time in the ice zones. Steel is somewhat easier to repair but plastic isn't far behind. The biggest thing I worry about with steel is galvanic reaction and stray currents.
I check the bottom paint for that (2 different colors with the "get me out of the water soon" color the first applied. Generally, I get out of the water every 2 years. Finding a lift that'll support your 65' will be far more difficult than a 32'.
A 65' boat would be about 50% wider than a 32 footer of comparable performance and manufacturer. Since slips are priced by beam and width chances you'd fine yourself renting a double-wide slip (like the cat boats do) or being on a T-doc. Both cost more. Also some mooring buoys have length restrictions and anchorages will get tight with a 65' boat. Maintenance costs are 3-5X higher than a 32'.
In most cases the bond is placed on the crewmember so I don't think size is important. Unless you plan to import the vessel into a less than friendly country.
Diesel is the way to go if a genset is that important to you. The need for one depends on the power requirements of the vessel. Gensets are more efficient (use less fuel/KWH) and are quiter. Properly sized they should have a 3-4000hour life. They also make a good backup. They make noise, require maintenance, regular operation, and cost a lot.
On the other hand they'll recharge the batteries in short order, don't require aiming like solar panels, and are far safer than the blades of a wind generator, not to forget less noisy. In an emergency a 115/220VAC powered bilge pump will push a lot of water and make operating power tools far more efficient.
If you're going cruising you've already decided that less is more. The quiet evenings at ancorage are so special. On the other hand if the Admiral wants to be able to do laundry and take long showers, then the genset may be the only solution. I got the best value for the money by putting in a very large bank of 6-volt deep cycle golf-cart style batteries, a 120A alternator, smart charger/regulator, and a wing generator. In 99% of the cases that was sufficient. If my boat was the movie or blender boat then I was forced to run the engine a bit longer in the AM to get things back where they belonged.