Q: OS X Mountain Lion

If you are going to upgrade, why not upgrade to El Capitan since Mountain Lion is no longer supported? You can order 10.8 using the link below. You won't need to do a clean install.

  Mountain Lion

Check to make sure your applications are compatible. PowerPC applications are no longer supported after 10.6.      

Application Compatibility

Applications Compatibility (2)

El Capitan 10.11 Compatibility information

Also check to make sure there is a compatible driver for your printer.

Do a backup before installing, preferable 2 backups on 2 different drives.

Back up your data, check your applications for compatibility, and buy the installer. Compatible applications will continue to run unless a problem occurs.

(143034)

You are welcome. Yes, you can go straight to El Capitan. Something else to check is how much RAM you have installed. El Capitan realistically needs 4 GB of RAM and more is better.

The 2 places I’ve seen recommended most to buy reliable RAM are below. I have purchased RAM several times from Other World Computing and have always been very satisfied with the product and service. They have on-line instructions on how to replace the RAM. OWC has also tested RAM above what Apple states is the maximum. I now have 6GB installed on a early 2008 iMac supposedly limited to 4 GB and noticed an improvement.

Crucial

Other World Computing