I'd say oil pressure and a voltmeter. Ammeter not so much since you have wire the ammeter in series with the cars electrical system requiring the usage of some heavy gauge wire. There is a fire hazard as that cable will be carrying your cars entire electrical load
There is no need to cut into the cable. You can use a hall effect amp meter sensor that goes over the alternator cable. Another way that works is to hook a millivolt meter up so that one terminal on the mv meter is connected to the alternator cable at the Alternator end. The other terminal of the mv meter is connected to the alternator cable at the battery end. The flow of current thru the cable to the alternator creates a small voltage difference between the two ends, and you can meausure this and calibrate it for acutal amps.
In other words, the cable from alternator to battery acts as a shunt.
Nice job for an Arduino
Here is an example to show the idea of the hall effect sensor, this one is too big and too expensive, but much smaller and cheaper ones are available
http://www.alliedelec.com/Search/Pro...DESC=RSS-100-A