Q: Whats Os X would you recommend me?

The most recent 10.11 El Capitan if the Mac meets El Capitan requirements noted here > Upgrade to OS X El Capitan - Apple Support

Unless you have copies of the several OS X versions that have been released over the years since your computer was made, you would need to use the installer discs that came with the computer to erase the disk and do a fresh installation of OS X. Thereafter, should you wish to upgrade, you can download the current version of OS X:

Upgrading to El Capitan

You can upgrade to El Capitan from Lion or directly from Snow Leopard. El Capitan can be downloaded from the Mac App Store for FREE. To upgrade to El Capitan you must have Snow Leopard 10.6.8 or Lion installed. Download El Capitan from the App Store. Sign in using your Apple ID. El Capitan is free. The file is quite large, over 5 GBs, so allow some time to download. It would be preferable to use Ethernet because it is nearly four times faster than wireless.

      Macs that can be upgraded to OS X El Capitan

         1. iMac (Mid 2007 or newer) - Model Identifier 7,1 or later

         2. MacBook (Late 2008 Aluminum, or Early 2009 or newer) - Model Identifier 5,1 or later

         3. MacBook Pro (Mid/Late 2007 or newer) - Model Identifier 3,1 or later

         4. MacBook Air (Late 2008 or newer) - Model Identifier 2,1 or later

         5. Mac mini (Early 2009 or newer) - Model Identifier 3,1 or later

         6. Mac Pro (Early 2008 or newer) - Model Identifier 3,1 or later

         7. Xserve (Early 2009) - Model Identifier 3,1 or later

To find the model identifier open System Profiler in the Utilities folder. It's displayed in the panel on the right.

     Are my applications compatible? See App Compatibility Table - RoaringApps

El Capitan for performance and security. Really does not pay to get behind in OS X. Stay as current as possible.

What non-system applications do you need to use?

If you have no need for the old PowerPC apps get up to date. If you must use PowerPC apps then you are limited to 10.6 unless the app is a Universal binary.

Universal apps also break with newer OS's so look at http://roaringapps.com for ideas of what may be compatible.

Hopefully you don't need those ancient apps, but it is a consideration I haven't seen mentioned yet.

Given that you’ve upgraded the RAM and intend to upgrade to an SSD, the current OS will run quite well. In fact, unless you already own a Mac and have downloaded previous versions of the MacOS, this version and the version that originally shipped with the Mac may be your only choices.