2GB of RAM can be found incredibly cheaply these days. So, it's an obvious choice for a first upgrade, but you might be surprised by how well the Mini 9 runs with less.
And if you decide to upgrade your SSD
eventually, installing a new SSD
on the Mini 9 is just as easy as installing RAM. The difference is that the SSD
has two screws securing it, while the RAM module has two spring-loaded tabs. Copying the OS over or reinstalling it from scratch is harder than the physical part of the upgrade.
Anyways, I hope your Mini 9 comes quickly, so that you can begin enjoying it.