Hello James,

It is often times best to upgrade your Operating System first. Installing Finale on an already upgrade OS will ensure the best possible install!

I agree with Kevin. OS upgrade first.

Upgrading is fairly smooth. Do not use the Chrome browser for the Finale upgrade process, however. Safari and Firefox are ok. Ordinary access to makemusic.com is fine, only the upgrade process has an issue.

Finale 25 will not overwrite 2012. You will find that 2012 will still work in a limited capacity if you turn Human Playback off but there are a number of other incompatibilities as well.

25 no longer requires Java but many apps, browsers and web sites do. Apple is no longer allowed to bundle Java with the OS (neither is Microsoft). You will get a Java error on upgrade that tells you to download the latest JRE 8 from Oracle—there will be a link when you fire up Safari. 

There will be a second Java error if you start up any 32bit app that requires it such as Finale 2012. This requires JRE 6. MM and Apple both have download links. https://support.apple.com/kb/dl1572?locale=en_US

Contrary to the error flags, you do not need the SDK (software developer's kit) only the JRE (Java Runtime Environment) — unless you are a programmer, of course.