So the spam was a problem back then, but I don't think there were any instances in which anyone who stopped spamming and started making decent posts was in any real any danger of getting banned. So if you take the premise that the OP bought the account and should have known it had a poor history, the way he could have resolved the poor history at the time would be to start making decent posts and he wouldn't be in any additional danger of a ban. Also, someone with a hundred posts (the number bill gator had when it was purchased) would generally not get permabanned as soon as discovered as it was posting garbage, it would generally receive a number of temp bans to give the opportunity to improve, so his risk at the time was he would receive a temp ban, and would need to make better posts moving forward, the later of which he did.
Plagiarism may have been common back then (IDK one way or another), but I don't think it was known to be a problem, nor known to be common.
All of this revolves around if Bill should have reasonably checked for plagiarism when he bought the account.
I would agree with your conclusion, and my opinion is that Bill should have reasonably checked for plagiarism/post quality. Im sure you recall how accounts were marketed back then, and post quality was always a factor. I remember playing with the account tool that everyone used and having it judge my post quality. While I agree that improving your post history is a good way to decrease the penalties, if you break a rule and you aren't caught you aren't punished. As soon as you get caught, you are likely to be punished. Will a moderator give you more consideration if you have 1 bad post for every 100 good? Certainly more so than someone with 10 posts with half of them being bad.
The risk as you said was that he would receive a temp ban, and he did. I'm not sure that it warrants a 60 day ban given the offense to contribution ratio, but thats if we operate under the assumption that there was only a single case of plagiarism in Bill's post history.