We managed to sign Amine Gouiri for 6m, including a 50% Percentage of Profit from next sale. For a player of his potential, that is a fair price. You might want to try to force out the additional clause, but you will have to bid a larger fee. Expect to pay at least 10M.
Amine Gouiri is a very good dribbler, which is considered to be his strongest attribute. He has a balanced and normal personality. Gouiri can play in more than one position, and you should be able to successfully retrain him as a right winger.
Amine Gouiri has the tendency not to get stuck into rough challenges. That can put his teammates under pressure and it will increase the chance of injuries. An attribute he will need to work on is his Consistency and the fact he is very one-footed. You should also know Gouiri is fairly selfish, which can lead to some unrest in your team. You should tutor him and hope his mentor will get rid of that negative player trait.