OSU went as far as Sullinger was going to take them & he wasn't going to get much better if he stayed, so leaving is the right move. I'm not going to miss all his posing, antics about officiating, playing the "I have to avoid fouling" defense, or complaints about the national media doubting the team. Don't get me wrong, Sullinger could play good defense when he tried, but 95% of the time he wasn't playing tough defense this year, and every game he was letting the other team get shots close to the basket easier than they should've been, not contesting high-percentage shots very well - you don't win national championships with that kind of defense in the post.
I don't know if Thomas will leave or not. His defense was not very good against the better competition, and he lacks the ballhandling skills it takes to be a wing in the NBA, so I doubt he'd be a first round pick. He might leave, but I don't think he will stick in the NBA if he leaves now, but you can't sneeze at the paychecks players get in Europe and other countries. Either way he's going to make a good decision.
If they keep Thomas and land Tony Parker (F/C recruit from Georgia), they won't miss Sully as much as you might expect next season. Sullinger is a great rebounder, and very good at scoring against smaller defenders, but struggled against strong & long big men, and he let his frustration get the better of him when things weren't going his way sometimes. I think Sullinger will be a bench player in the NBA, but he should have a nice long career because he will find a way to get his share of points and rebounds & NBA teams need quality bench players like him.
I think Buford is going to be harder to replace than people think because he was the best shooter on the team and played good defense, and the guys they might replace him with all shot poorly from 3 this year ... I don't know how many (if any) of them will be able to both shoot 3s well as well as defend well.