Damp Proofing

I live in a 2 bed mid terrace and have soft plaster on front wall and also on one inside wall. I am guessing that I need damp proofing - how long does it usually take just to do the damp proof as I am selling house at the moment and wondering if it is worth getting this done myself or knocking some money off the asking price? Also can anyone help with giving me a clue as to what I put in the boxes to get a quote lol There is no "I need a damp proofer"!