Well, I was in a similar situation as you a few years ago, but on a much smaller scale. I picked up a used JBJ30g AIO from a guy off craigslist. He told me the tank was dirty, covered in algae and coralline, but assured me it did not have any major scratches. I drove the 2 hours and picked it up and yeah it was pretty dirty looking, but for the $150 bucks I threw at it, I figured I could clean it up no problem.
So I bring it home and immediately started a vinegar bath, let it run a few days and, of course, it melted off all the junk on the glass. That's when I knew the guy had lied to me.... It had a ton of scratches, not as many as you have, but tons of smaller to medium scratches, some minor, some deeper. I researched like you did and read up on ways to polish the glass. In the end, I decided it was not worth the effort for a 30g tank to attempt it. So, I set it up and the water hides most of the scratches and it does not bother me nearly as bad as I figured it would. The scratches eventually fill with algae so they become even less visible unless you focus on the glass and not what's behind the glass.
At the end of the day, you will just need to determine whether you will want to have the annoyance of scratches or have peace of mind and just buy a new one. You eventually overlook the scratches for the most part, or at least, I did. I will never purchase a tank that hasn't been pre-cleaned again, though!! Also, be careful filling that large of a tank up sitting on the floor of your garage, especially with that moving blanket or whatever it is laying beneath it. If it's not perfectly level, or at least within a certain tolerance you can blow out a seam. I'm sure you are draining it right away, but still be careful!!