In today's market 100M is a good deal for a player like Griezmann.
We can argue about signing a younger player because you can get more out of him longterm, but short- to midterm Griezmann will be the best choice for 100m if Valverde wants to continue with the current system. After all Messi has done for this club we owe him to buy Griezmann because he would be the perfect addition to our forward line. He would do well while "running into" Leos passes. He is also good at playing one-two and links up well with teammates. He could easily play that Villa/Henry role.
Given our recent history with French players I'm pretty sure Griezmann will be a success here.