calculating input resistance of darlington pair (TIP120) from datasheet values
After Searching for a while, i was only able to find solutions for
  discrete darlingtons,(these required knowing parameters for each
  transistor (like the hfe,hie that are missing in tip120 datasheet).


The data sheet is particularly good for this device. Here are a few snippets: -



Given that hIE is the input impedance and hFE is both stated and graphed I'm not sure you are looking deeply enough at the data sheet. 


  I need this parameter to perform the calculations concerning the AC
  Analysis of this circuit


As with virtually any analysis I would urge you to use a simulator to double check any hand calculations you may make. Sims are free and good (usually) and far more accurate than a hand-calculation.