I would go with the Oil pressure first.
That one has saved my engine from a catastrophic failure.
You can also see with the oil pressure when the oil is starting to get up to temp.
On startup the pressure can be about 5-6 Bar on idle. But when warm it's around 1.5 Bar when idling.
An oil temp gauge is usefull too.
The only use I have for it when daily driving is to see when the temp is good for bashing
The S15 gets up to temp pretty fast, so if you just wait 10min after starting you would most certainly be up to operating temp.
It would be a good safety measure for trackdays though.
Best is to get them all
What you need to do Nicely is to get rid of them centre air-vents and stuff em with gauges :thumbs:
Edit:
I know some companies here in Sweden who sells those gauges with the Silvia S.
I could look up their homepages if you want for prices.