if you do the timing belt (just a good idea with that many miles and years), do the tensioner just to be safe. and be sure to use a deep well 12-point 10mm socket. if you use a 6-point, you will most likely round that bolt and be in a world of pain and misery.
personally, i have had pure bad luck with NGK plugs - 2500 miles and they're losing power. i would recommend factory Denso plugs personally - ran them for over a year straight with zero issues of any kind.
oil pressure sending unit is easy to test. start the car. look at the oil pressure gauge. rev the engine up to 6000rpm for 2 or 3 seconds and watch the oil pressure gauge - if it moves UP in oil pressure, then i would say its working fine.
if/when you do the timing belt, you'll be able to inspect the front main seal, the cam seals and water pump. if the water pump has failed, you will see water spill out of it from the 'weep hole' which, i believe, is right on top. it is an easy part to replace overall, and is not too expensive either.
so, if you are in question/doubt of the water pump, just replace it.