For the pump you get every single part, sometimes you might get cheaper after market parts, so there is usually no need to change the whole pump.
To solve your problems:
1) Rinse the seawater circuit and replace seawater with tap water.
First open the seawater strainer and fill it up with tap water to displace the seawater through the saildrive leg.
Then close the seawater cock at the saildrive.
Start the engine with open seawater strainer and replacing the water level in the strainer as the seawater pumps sucks it through the engine to replace all seawater with tap water. Always keep the water level above strainer level. Don't run the seawater pump dry. 10 liters of tap water should be enough to rinse the whole sea water circuit
2) Servicing pump
The cover of the pump should be perfectly even without any traces of wear and tear. If one side is brittle, you can turn the cover inside out (and remove the paint from the outside first with a light sanding). Replace the O-ring, giving it a light smear of silicone grease. The marks around the O-ring on the pump body should not be a problem.
To remove the impeller use a special tool and not a screw driver.
It is useful to remove the lowest step of the companion way to get good access to the pump.
If there are leaks on the back side of the pump (seawater or oil) you need to remove the pump from the engine. There are 2 sealing rings (one for oil on engine side, another one on pump side for sea water), which can be replaced. Check the pump shaft for wear on the position of the seals and replace the shaft if necessary. At the engine side there are two bearings, which could be changed.
I did the repair of the pump when my motor oil was drained, so i don't really know, if you can remove the pump without spilling the engine oil.
After reassembly of the pump renew the outer surface with the green VP paint to give everything a nice look
Before you start with this job i suggest you to study the service manual and the spare part drawings provided by VP.
The whole job is pretty straight forward and no rocket science.