When buying a house many people will do a drive by the area first and get an overall impression of that house from the way it looks on the outside. So do the best you can to make this an inviting starting point so they then want to come inside and buy your home. Painting an existing front door will not only freshen up the front of your house but add colour and interest. Finish off with some hanging baskets and coloured pots, have a general tidy up and hey presto you will be done.
Sometimes a colour you would never have thought of can look amazing, so below is a video of different door styles and colours to give you some ideas.