Aside from Clinton, only Andrew Johnson back in Reconstruction days was actually impeached. Nixon resigned prior to the full House formally voting on the articles of impeachment, but as has been stated here senior Republicans assured him that the h
House would approve the articles.
As for conviction in the Senate, the released White House tapes, which showed conclusively that Nixon had engaged in obstruction of justice, were sufficient evidence to garner the votes for his removal. There is really no question about this.
Presidents' reputations are revised all the time. Ford is a prime example. His pardon of Nixon was reviled at the time. Many historians (and not simply conservative ideologues) now see this as the best course for the country. The vendetta against Clinton (as slimy as his actions were) showed how much damage to the country prolonged political wrangling can do. Ford's action let us move on.