Hang 250 kg (550 lbs) on concrete wall
I'm not sure I understand your concern about putting it on a pedestal. Doesn't your plumbing need to match up with the inlet/outlet on the heater no matter how you mount the thing?
If you have fixed hard lines that you're trying to match up with, just build a pedestal to the proper height so its connections meet up with your existing lines, set the empty tank on it, use the included fasteners to also fasten it to the wall (overkill never hurts), then finish the plumbing connections.
If the lines need to run into the bottom of the tank (quite annoying, IMHO), build an open frame on which to set the heater without interfering with the location of the lines, then do as above.
If you've got flexible lines, then your pedestal height is mostly subject to how long a flexible line you've got/can get, and how high off the ground you want the tank. Again, follow installation from above.
Having something supporting the weight from below, whether wood, concrete block or steel seems to me to be the safest, surest way of doing it. Using the wall-mounting hardware provided by the manufacturer in addition simply adds to the security.