Q: What Operating system for my old iMac (Early 2009)?

This link shows the system recommend requirments.

Upgrade to OS X El Capitan - Apple Support

How much RAM is installed in your Mac?

It sounds like it should be able to handle El Capitan then.

Your first step should be at one if not two backups.

I would also recommend that you holdout doing the upgrade until you have or can make a break in work. Upgrading under the pressures of work is alway a poor time to do it. I have had to help bailout more guys after they hosed up the upgrade because of the pressure of work.

You can still get OS X10.8.5 Mountain Lion, but it is no longer free.

http://www.apple.com/shop/product/D6377Z/A/os-x-mountain-lion

However, if you had downloaded this prior and had this on your system, if you go to the purchases tab in the Mac App Store and Mountain Lion is listed in your purchases tab, you can redownload it and reinstall it for free.

Everyone seems to always push the latest OS X, but it doesn't always work out for users and it isn't always best for older Macs.

You have options.

You can reinstall OS X 10.8.5 Mountain Lion, if you choose, OR upgrade and take a chance on the recent version of El Capitán.

Check to make sure any apps you were running are still compatible with OS X 10.11.6 El Capitán here.

http://roaringapps.com/

Good Luck!