I recently purchased a 2 year old site which is earning nice revenue and does fantastic in the serps and was worried about that too. I have purchased a few sites before but nothing this expensive. That was nearly 2 months ago and revenue is as expected and I have no change in the serps.
I suppose if you think about it, Google's algo has positioned the site highly for a reason. Meaning your content/backlinks are fine. So it still provides value to users, just because you buy the site, I don't think Google will "punish" you.