AVR Selection

The governor is more important.

As Tornado wrote, the governor (speed control) is more critical. For the AVR itself, just to synchronize...

1. It must give a stable output voltage, that is the voltage must not be cycling around, slowly enough to be visible on a panel meter or quickly so that the meter hides the fluctuation. Stability is needed on-load, in parallel also - to allow use with many makes of generator an AVR usually has adjustable gain (about 20:1 range) and machine field time lag compensation, typically by variable resistors marked "gain" and "stability".
2. A voltage adjustment for machine voltage of +/- 10% of name-plate value to allow adjustment to match the running system before connection.
3. For convenience, this voltage adjustment usually has to be remote from the machine. The conventional way is to have a motorized potentiometer [or an electronic equivalent] which can be driven up and down by two remote pushbuttons or a 3 position rotary switch Lower - neutral - Raise, spring-loaded to neutral.
4. The speed of change must be compatible with generator etc dynamics - about 30 seconds for 10% voltage change suits large machines.
5. The voltage must change smoothly, without big steps due to a coarse wirewound potentiometer or a digital reference - less than 0.1% steps is usually OK.
   
6. The up-down voltage control method is compatible with most automatic synchronizers.
7. A separate "Preset to normal voltage" input may be convenient, used automatically during run-up to get voltage "nearly right" even if the operator left voltage at max or min on last run - particularly if a quick connection is needed.

If you look around in the websites of Caterpillar or Cummins you should find AVR specifications in the "customer support" or similar areas e.g.

www.cumminsgeneratortechnologies.com/en/products/​stamford