Welcome aboard and congrats on the new to you boat
Get a mercruiser manual which can be found under Documents of this site and follow it service recommendation it also has troubleshooting tips.
For the tach, rev counter on the back is a switch for 4-6-8 cylinder try rotating it back and forth to see if that corrects the problem. Also verify the S terminal is seeing a pulsed signal between 2-10 volts DC.
The oil pressure issue may be a stuck gauge so try disconnecting the wire on the S terminal. If still reading 40 the gauge is faulty.
1997 2400 SCR
5.7 Vortec / Bravo 2