Overall - if the car is running - all voltage will be regulated as the alternator + battery form 1 system, however I would connect as close to the battery as possible - which would mean at roughly the ignition switch, or you could splice in at the fuse panel - doesn't really matter. Also, if I remember right, the Voltage gauge only has the one set of leads + and - (well that and the bulb). You can get power for the oil psi gauge there as well.
Isn't 0-150 psi a bit wide range - might 0-100psi be better? I only as because in my e36, oil pressure is rarely ever above 60psi (possibly due to sensor location/connection). If the oil system never sees about ~80ish PSI, the 0-100psi sender+gauge would give you the best resolution.
@Bill Williams - good to know!