Compression test is a good thing because it tells you much about the mechanical fitness of the engines internal parts without actually disassembling it. If it's a non-running bike then the owner should be supportive of you performing a non-destructive test imho. If the compression test numbers are within spec you and the owner now know the problem is likely electrical or carb related which is a much less significant repair issue because you know the valves, pistons and rings are all doing their thing correctly.
The problems with a 30 to 40 year old bike is not just milage, it's the age and deterioration of every piece of rubber on the bike, potential mouse damage, rust corrosion and condensation if the machine was stored in less than perfect conditions and the difficulty in obtaining replacement parts.
"partially restored" is not a good thing, that is a minus, it represents time &/or money invested by the original owner that needs to be recovered, it was not a successful restoration because they ran into a stumbling block and now they want to bail on the project. As for the cost of the bike :/ not a lot to go on having not seen it, but at least they aren't asking twice the original list price of the vehicle.