I looked at the pictures again, and at $23k, it's really hard to say. In one way, I'd say it's worth it; I had a friend here in NE Georgia sell a driver-quality 351 '69 Mach for $25k just a few weeks ago. The body on the car you're looking at looks pretty nice and original. It's clear that the car has had a "rattle-can restoration" on the engine bay (wiring harness and motor mounts have overspray). But what gets me too is all of the little stuff. The door ajar switches have been painted over. The interior is serviceable, but is missing the console (and that's not cheap to replace). The quickie paint job turns me off the most, and it makes me wonder what else has been skimped on in the past when work was done.
What's your ultimate goal for this car? You could probably spend $10-15k more and have a much better car to start with if you're going for a concours restoration. Something else to think about is the rarity of the car; it's a well-optioned car, but it's not super rare or optioned out. If you're going to dump $20k in a restoration, is this the car that will get you your money back when it comes time to sell it? Just something to think about. And I don't mean to bust your chops--I'm in the same boat with the '69 Mach I have now. It's a garden variety 351W/auto car, and I've got enough in restoring it that I probably could've bought a big-block car. If you're doing it for fun, then that's one thing, but if you're doing it to make money, I don't see all that much profit on this car even at $23k. The 390 is desirable over the 351W, but most people with money to spend are going to skip straight to a 428 car.