I reckon oil pressure is a must. oil temp - probably if you are racing it (circuit mostly, it's harder to get the oil to ridiculous temps in a drag race). On the oil pressure thing - if you are racing it gets hard to monitor a dozen guages, with traffic around you, looking for braking points, you name it. You can get the old style 'warning light' type oil pressure switches, and wire that to a big red light on the dash, and possibly a small buzzer. BUT instead of using the factory sender, which probably activate if the oil pressure drops below around 7-8psi (I don't know exactly), instead you can get ones that will activate at 25-30psi. If you are dropping oil pressure during a race to the point you get down to 7psi, you'll likely be damaging stuff. If you get to around 25-30psi and the warning light/buzzer both go off, you can shut down the engine and the damage will be relatively limited (of course there's still going to be 'some' damage - whatever caused the oil pressure to drop in the first place, unless it just got too hot, or perhaps the oil pressure relief valve jammed open or something?) I would actually suggest that this buzzer/light/sender would be more valuable than the guage, in a racing situation at least.

Ideally you'd run them and a guage/sender unit. THen you can monitor oil pressure around the track. It can be very helpful to pinpoint where and when it might drop oil pressure and it can then be used to work out what to do as far as improving teh sump design (for example) to prevent it happening any further