This is probably better asked as a "Short Answers To Simple Questions" post. Here is what Chris DeRose writes in his book The Presidents' War: Six American Presidents and the Civil War That Divided Them:

James Buchanan believed Lincoln was “a man of kindly and benevolent heart,” an impression that never changed. He wrote, “The ways of divine providence are inscrutable, and it is the duty of poor, frail man, whether he will or not, to submit to his mysterious dispensations. The war—the necessary war, forced upon us by the madness of the rebels—we all fondly hoped was drawing to a triumphant conclusion in the restoration of the Union with a return of friendly relations among all the states, under the auspices of Mr. Lincoln. At such a moment the terrible crime was committed, which hurried him into eternity; and God only knows what may be the direful consequences. I deeply mourn his loss from private feelings, but still more for the sake of the country. Heaven, I trust, will not suffer the perpetrators of the deed and their guilty accomplices to escape just punishment.” But “we must not despair for the Republic,” offering his encouragement at the presidency of Andrew Johnson, whom he had “known . . . for many years.”

The quotes of Buchanan that DeRose pulled from are found in The Works of James Buchanan Vol. 11, pages 382-85. You can see the original letters here.

Elsewhere in those letters on those pages, all written between 18-27 April 1865, Buchanan called Lincoln's assassination a "deplorable event" and "a terrible misfortune to our country". He hoped that "God, in his mercy" would "ward from us the evils which" the assassination "portends, and bring good out of this fearful calamity".