It sounds to me like the head gasket or a cracked head if the engine is hard to start, turns over a little slowly. Water in the bores means the compression will be significatly raised for cranking.
I might suggest that you don't try to drive the car around until you have sorted the issue ,or risk bent rods or bearing issues which will make the whole deal very expensive.
If your battery is on the way out (as mine is) every morning the radio will have lost its presets and you will find that the power windows will go slower than a wet weekend. A battery that is not holding it's charge etc is VERY obvious so I would be pointing at head issues.
The spark plugs are what I'm reffering to, if you are a newbe then it would be time to have a mechanic look at it alternatively hook up with a local skyline australia forum member or two who is technical and offer a few beers to have a look. I'm sure there are a few guys in your area.