Manolas would be a great option imo. Not very expensive and still quite young at 27.
De Ligt should be the first choice, but he will most likely go to Barca.
Koulibaly in terms of value should be the best CB in the world - He is still young, is very fast and strong and is really a great defender
Before you all start bashing me I said in terms of value which means Chiellini`s age reduces his value while Ramos isnt young anymore himself...