Logged on to see if any news of Mina's fitness but got quite interested in reading everyone's comments.
As a Colombian who has followed his career, I think he will be fantastic and possibly exactly what you need. I value his leadership so much that it is perhaps an even greater attribute than his skill – which is not to say he is a slouch. If he has a weakness it is pace, but I wouldn't call him slow (just not as fast as Davinson). He is incredible in the air, on both attacking and defensive set pieces. Can pass the ball and bring it out of defense in a cultured way, won't waste it. Huge presence. Confident. Scores goals. Lots of them for a defender!
Anyone who says his 3 World Cup goals were out of character hasn't followed his career. Yes 3 in 3 is absolutely incredible for any player, nevermind a defender. But his record for country is 6 in 15 (better than most strikers), and 6 in 28 for Palmeiras isn't bad. Colombia resorted to set pieces after James got injured, knowing his prowess; and it worked.
What hasn't been mentioned as much is that Colombia only conceded 1 goal in the those three matches he played in – the penalty against England which had nothing to do with his defending. He and Davinson Sanchez are excellent.
As far as the Davinson comparisons – Mina is not as fast, but he has greater presence, confidence, is more vocal. Together they will be one of the best pairings in World Football by the next World Cup barring injury.
The only blip on Mina's career was his introduction at Barca. He didn't adapt. Arrived in January after about a year of negotiations. The manager had requested someone with European experience to come in and challenge for a starting spot immediately and instead got Mina from Brazil, which did not please him. Mina didn't get a chance for a few month, becoming demoralized... and then played poorly when finally called upon. You can argue the pressure got to him; but he has approached every other challenge (scored on his debut for Colombia; took like fish to water at Palmeiras one of Brazil's biggest clubs; 3 goals in 3 World Cup games) with confidence and the environment at Everton should be just right for his first real run in European football.
I think he is a top, top signing, will improve those around him, and you will get a lot of money for him when he leaves. Man United could have used him too.