If we go for Koulibaly it would surely be to play with Godfrey.
That would mean one of Mina and Keane being sold. He'd be an upgrade on both of those players.
He's a month older than VVD, so i suppose the question would be similar to whether you would take VVD (whose also 30 before next season) at a cost of around £40mil + enormous wages?
On the positive side we get a superstar centre back for 3 or 4 seasons (who will decline). Godfrey would probably improve as well through playing alongside him.
Id counter that with Tapsoba at Leverkusen being available for £44mil buyout fee who displays alot of the same characteristics as Koulibaly and is only 22.
There are others like Kounde (Sevilla) who have everything though are a little small at 5'11 (but a Cahill type leap). He'd cost about £50mil, down from £65mil ish City were quoted last summer.
Also Bastoni at Inter who is amazing. Both of those players can play the conservative cover role with Godfrey moving forwards to snap the attacker.
If it was me, id go for the younger options...for the kind of money and where we want to be then we need to go for someone top class who will become world class...not someone on the downward slope.