Hi,
I'm Máté Marozsán from the GRAPHISOFT and we don't have too many experiences with the new Mac Mini yet, but based on the reviews and the available hardware options it can be a good solution.
But I would like to encourage you to take into consideration that your team might grow by time and the hardware you pick won't be enough. It is important to know because upgrading the the Mac Mini is quite limited (especially within the warranty period).
Mac Mini 2018 recommendation: If the Manager and the Server is installed on the same computer then choose a 6 core processors (i5 6 core 3.0 Ghz, i7 6 core 3.2 Ghz).
The RAM need always depends on the number of the active users, projects and the size of the projects. In general our recommendation is minimum 16-32 Gb RAM. To have a proper value we have a calculation method that worth checking. Check the size of the active projects on the Manager UI and use the following (example project sizes are 3 GB and 4 GB): 1,5*(3GB+4GB)+2GB=12,5 GB. In this case regarding the need of the running applications and the OS: 16 GB is a minimum.
You can find more details about these calculations and recommendations below:
https://helpcenter.graphisoft.com/knowledgebase/64444/
But if the studio or the size of the projects grow potentially the consider the 32 or 64 GB option to have the performance that won't cause you issues in the future.
In this case considering the price of the Mac Mini with 64 GB of RAM, a powerful PC might be a better (less expensive) option.
I hope it can help you!
Kind regards, Máté