Probably none with that iMac, at least as far as using it as a monitor:
Feb. 2016 https://discussions.apple.com/thread/7454088?answerId=29761388022#29761388022 - "Whether or not you can use your older iMac as a secondary display for your new iMac (known as Target Display Mode) will depend on the specific model. "
Use your iMac as a display with Target Display Mode - Apple Support - https://support.apple.com/HT204592 - iMacs late 2009-Mid 2014
Alternative for unsupported models: Virtual Network Computing.
-iMac models you can use as a display
-Cable you need
-Port you need on your other Mac
iMac (27-inch, Late 2009)
Mini DisplayPort to Mini DisplayPort
Mini DisplayPort or Thunderbolt
iMac (27-inch, Mid 2010)
Mini DisplayPort to Mini DisplayPort
Mini DisplayPort or Thunderbolt
iMac (Mid 2011-Mid 2014)
Thunderbolt
Thunderbolt
iMac (Retina 5K, 27-inch, Late 2014) and later iMac models can't be used as Target Display Mode displays.
ALTERNATIVELY:
How to use target disk mode to move files to another computer - Apple Support