The signing of Koulibaly - from Racing Genk - has boosted the Spaniard, who hopes to make further additions to his Napoli squad during the close-season.
Napoli finished third in Serie A behind Juventus and Roma, and although they were 24 points behind Antonio Conte's champions, Benitez took plenty of positives from their campaign.
"Napoli is definitely taking steps forward," he wrote on his official website.
"This, obviously does not guarantee success but undoubtedly is helping to get it in the future. We are still looking for players that are suitable for our project and to ensure the present and future of our team.
"I remain in periodic contact with the president and his collaborators.
"Together we analyse the team and the possible reinforcements. The arrival of Koulibaly means greater competition in defence, another step forward, but we don't stop there."
Napoli have also been linked with moves for Lazio midfielder Antonio Candreva and Barcelona's Javier Mascherano.
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