Ok I'm going to try replacing 744, are there any detailed instructions?
Do I shut the power off to the unit?
Yes, always a good idea when working on a unit.
Or do I just need to shut off the water line?
Shut the line off then unscrew the fitting at the water valve. Do this slowly.
Let it drip/drain into a shallow pan which you can empty. Once the pan is full you can finger tighten the fitting so you can empty the pan. Will minimize the amount of water you will have to mop up.
Also it seems that items on the top shelf don't get as clean as lower, could this also be not enough water in the unit?
Also do I need to use any silicone sealant, plumbers putty on the connections when I replace the water intake valve?
I usually do a couple wraps of PTFE thread tape on the treads.