I don't have a Synchro but I do have an 89 Westy 4-speed. A few additions
to your list: First, try to determine the owners attitude toward vehicle
maintenance. Can he tell you the last time he changed oil? Does he keep
maintenance and repair records? Does he know how often the plugs have been
replaced?, etc. Does he do repairs himself? Ask for repair records and
other maintenance records. Ask "what repairs are needed on this vehicle?"
It can't hurt to ask.  The owners attitude and knowledge tell me a lot
about what I may face in the future with a used vehicle.

Other things:
2.Check condition of canvas in pop-top
3.Run the heater blower for at least 15 minutes and listen for squeaks and
rattles indicating need for a new blower. This is an all-day job.
4.Body rust. Yes, check the seams but also check the undercarriage. Rust
under the body is just as important.
5. Check condition of collant hoses in the undercarriage.
6. Check condition of gas lines. Look for any weaknesses or rust.
7. Check wind noise at 75mph. These are noisy vehicles and any worn rubber
linings on doors or windows can make conversation almost impossible.
8. While doing the highway drive, put the brakes to a test. Does it stop in
a straight line when the brakes are applied hard at highway speed? Be
careful!
9. Look at the condition of the air cleaner and smell the oil on the
dipstick

Good luck. Dan Roberts