Maybe this helps in making a decision: https://www.mendix.com/evaluation-guide/app-lifecycle/app-development#enable
If you mean by ‘design’ the look-and-feel of your app you probably need some extra tools like and IDE and Calypso.
If you mean developing your app, it all depends on how complicated your apps needs to be. If you don't need integrations with other systems of custom java logic, you might be able to suffice. But in my opinion with any serious app you can't work without Studio Pro in the long run.