Must have a tach. Additionally, I always install AutoMeter Sport Comp, 2 5/8" mechanical water temp, oil psi, vacuum and volts in every vehicle I buy, minimum. Some, like my race Buick, also get trans temp, oil temp, and fuel psi. Actually, that one got a fuel level gauge and tach as well because I ended up building my own dash. If you have forced induction, then you would obviously substitute a boost/vacuum gauge for the vacuum gauge.
Take care,
~Chris