I am not sure a voltmeter is worth the space on the dash. I have one as standard on my 911 and quite honestly it’s a waste of space for what it tells me.
An AMP or “ammeter” would be more useful as that shows the rate of charge / discharge to / from the battery and from this you can tell if the battery is being charged and more importantly, holding its charge.
They are more complicated to fit, as they need to be in-line with the dc output from the alternator. Having said that, there are digital versions that have a remote sensor unit fitted to the alternator output and a separate display for the dash.
I think you were also asking about an oil pressure gauge.
There are 2 types. The capillary type has a copper tube connected to the instrument and normally “T’d” off the oil filter housing where the standard low pressure switch is fixed.
The second type is electrical where the sensor goes as above and is connected to the instrument by a single wire. This type is easier to fit.
This is the sort of thing, not a recommendation, just to explain what I mean
http://cgi.ebay.co.uk/SWOOSH-GAUGE-...8QQihZ019QQcategoryZ43120QQrdZ1QQcmdZViewItem