My Yanmar (and Norsea 27) sat on the trailer for 4 years during one stretch of 5 years when military assignments took me here and there and I had no one to look after the boat. With the old fuel and a fresh battery, the engine started within two revolutions. Turns out the aluminum fuel tank was rotted and there was water and a biology experiment in the tank, but the "clean" fuel in the pump, lines, and injection pump was enough to test run the engine for a couple of minutes.
My recommendations are based on fighting this fuel system and neglected engine (I know, my fault) for a couple of years, which now makes me nervous about an idle, neglected engine, so I start it regularly while the boat is on the trailer. I would do the following (not in chronological order):
I don't think your water pump impeller needs inspection and repair. The boat has never been in salt water, so the engine was run on fresh water, and probably laid up to prevent freezing, and the impeller is basically unused and never exposed to UV. You can't put a pressure source on your water intake. That will flood your engine through the exhaust manifold, so when you are ready to run the engine, disconnect the supply to the water pump and put a length of half inch ID hose on it, and the other end of it in a 5 gal bucket on the deck in the salon or head. Put water in the bucket and let the pump lift the water to the engine. Fill the bucket with a hose as needed. I usually fill the pump supply hose by partially filling the bucket, putting it in the cockpit, siphoning water into the hose, then moving the bucket inside. I put a garden hose in the bucket and adjust flow as needed to maintain the bucket level while running the engine.
If the starting battery hasn't been replaced, do so.
For the fuel system, buy a six gallon portable fuel tank and the lines and fittings to fit hoses to the vent and to the supply to the inlet to the fuel lift pump for the engine. Disconnect the installed fuel tank at the inlet to the fuel lift pump and connect the tank, with a bulb pump in the line to use for priming. Change the engine fuel filter and prime the filter by cracking the vent on top of the filter while pressing the bulb. Open the vent when the bulb is pressed and close it before you release the bulb to press it again, as needed. Get all the air out of the fuel supply.
Check that the engine fuel control cable operates smoothly. The only internal issue I had with the long layup of my engine was that a little rust built up on fuel rack on the fuel injection pump. As a result, the throttle control and governor would not perform reliability. The FI pump and governor gave fair service for a good while after the engine was placed back in service, and I only found this much later, but you can check cable smooth operation without disassembly now, and pull the governor cover and check the pump rack later when you have time. The FI pump rack is a precision ground fit in the pump and must slide almost friction free.
Change the oil and filter.
When you first try to start the engine, use the compression release and spin her good, then release it and see if she starts. My guess is she will. If the engine doesn't start after about 20 seconds of spinning, stop trying. If you spin the engine longer than that, you risk pumping water into the engine...exhaust is needed to expel the cooling water via the exhaust.
If the engine doesn't start, prime the fuel system again, up to the FI pump inlet and the excess fuel line that comes back to the fuel filter from the injectors. That primes both sides of the pressurized fuel system with fuel at low pressure, and the FI pump should do the rest. Repeat as needed. There is a bit of art to priming. You may not get it done right the first time.
Try to start the engine again. When it is running, be sure to check that the exhaust pulses cooling water out the back of the boat.
When it is running, I would run it at relatively low speeds for a while, building up RPM pretty slowly, because the cylinders/pistons/rings could have a little bit of corrosion and the engine really hasn't been run long enough to break it in.
I would run the engine from the portable tank long enough to ensure that fresh fuel was everywhere in the engine fuel system. Then I would tackle the fuel tank and filtration system as a separate issue.