Toure, Batemon and Milan are musts. I like Pleta but he's replaceable, the other three would be a lot harder. I'm very bullish on Toure, I can see him making a big jump to star status in his 2nd year. TJ tends to keep a lot of his guys so I'm confident that the young core and Milan will be back although if he has a big tournament he might go pro.
Edit: And Buchanan. He fits well here but I also think he's replaceable if he decides to move on.
TJ has said himself Buchanan is the PERFECT 5 for his system.
6'10" former top 100 recruits with extremely quick feet do NOT grow on trees. He will also be a senior with his best basketball ahead of him. He will be a priority to bring back.
As an aside, in regards to Pleta, he missed the majority of the offseason in Ames. Give that guy an offseason of strength and nutrition and post work with Coach Green his ceiling is extremely high.