I wouldn't think that a change of domain ownership would harm a site's rankings. I would be careful buying a business on the strength of its Google rankings - as we all know things can change quickly - I would check where all the links to this site come from.
There was a situation about six months ago where I was looking to pick up an expired domain name, and I saw that I would likely get nothing out of it since the backlinks wouldn't be counted. In fact, the previous registrant re-registered the domain name, and now has no backlinks in Google. I guess I was confusing this situation with that one.