A pressure washer will not damage a liner. Select the proper nozzle. If using a pressure washer with the appropriate tip damages the liner, it wasn't much of a liner to begin with and should be replaced anyway. Better to find out now than to go to all the trouble of rehabilitating it only to find that the liner is way past its prime and is needs to be replaced..
As an aside, most EPDM liners are good for 25 years, depending on thickness.
It looks to me that there's stone coping around the perimeter. As others have mentioned, run-off (particularly if you intend to fertilize the surrounding plants) will cause you a lot of problems (ie. algae blooms). If you are determined to keep it at ground level, I would suggest installing an overflow pipe (for heavy rain), and a drip irrigation system for the surrounding plantings (will eliminate run-off by watering with the hose). A siphon system will allow you to put fertilizer through the drip irrigation lines.
It sure looks like you've got your work cut out for you.