All those things you mention above should be done. Basic maintenance as Nel621 says.

Here is a good maintenance list for high mile cars I pulled of a post (sorry, I can't credit who came up with it)

Replace all of the sensors (MAF, ECT, O2, don't worry about the knock sensor) if they are original
Flush coolant, trans, diff, steering, and brakes.
Install in-line filter (Magnefine or the like) on the power steering and trans (if automatic).
Fuel filter
All rubber lines - coolant and fuel especially
Ignition Tune-up (plugs, wires, cap, rotor)
Clean out the PCV system, throttle body, and IAC
Clean up the fuse block and replace any of the junky plastic-bodied fuses with ceramic
Check/adjust valve clearance

Aren't there wiring harness issues for 80s 240s? I've only have 91 and later bricks, so I'm not the expert there.

A car from the original owner who is a Volvo mechanic would be hard to pass up.

The only issue I see is it being in storage for 12 years and started once per year "to keep everything lubricated". Proper storage would have been to fog the cylinders with a light oil oil and rotate the motor by hand every few months to avoid the piston rings putting rust lines on the cylinders. Sure, it is a Volvo redblock and will run, but I'd guess there are ring lines that may cause early blowback. My guess is the owner being a mechanic will say otherwise, but if he were a "good" mechanic he'd know this... Steel rusts without oil protection. ???