Does anyone have experience buying a domain with these problems?

I have the option to buy a few domains that are in my niche and could work either as part of a PBN or maybe money site. They have been online for about a year, they are simple, relatively crappy sites run by a hobbyist. I think I can turn them into something useful.

The problem is that their robots files block all user agents. I guess they never changed it when moving from a development environment to a live environment. This also means the domain isn't indexed and not in the web archive.

Would buying these domains be the same as registering completely new ones?

I am curious about that, too. If they are indexed then they probably would not be identical to buying new domains.