I would like to suggest you to take the last Mendix 9 version avaiable, at the moment it is 9.2.0
Also you can check the following link to check the release notes:
For the future, depending on your project needs, it's best to check out the LTS / MTS versions on the page for downloading Studio Pro. If you're looking to go live very soon, it might be worth considering using a version of Mx8 instead of 9. If you don't plan on going live for a long time, just take the latest version of 9 until an LTS / MTS version of that is made.