There are many people on this site far better placed to answer this issue than I, but here's a view from someone with a high mileage car that doesn't seem to have sat still for too long.
First, if you're about to become the owner, then it's a bit late for 'is it a waste of time.' The question, 'is it a waste of money' might be more relevant, in which case it depends on what you're paying. You seem quite certain about the provenance, so it should be OK.
I imagine that you might need to consider carefully how you approach your project if the car has not been run for 20 years. You could do a lot of damage by trying to start her too soon if she isn't already running. There is a lot on this site about starting cars that have not run, and the general consensus seems to err on the site of taking it easy and ensuring that key components are up to the job: FI pump is not frozen (old fuel gone to varnish); fuel tank free from old fuel and not rusted; fuel pump OK (varnish again); engine not frozen; hoses/lubes in good order, etc. As for the mileage, provided the car has been stored with a degree of care, and provided you approach the start up carefully taking advice where you feel you need to, I can think only that a low mileage engine wouldn't be a bad thing. A low mileage car that has been stored for a long time might need additional TLC (there's a lot of rubber on the chassis: engine mounts; subframe mounts; suspension, etc) that might have turned to dust and which could need work, together with the brakes, but again I would imagine that much depends on how it was stored and how much you're paying.
Either way, there is a ton of help here and lots of great stuff to be found. I hope your car works out well for you. Don't forget to post some photos.
JH
