How much power does it really make?

DS18 rates it at 100 watts RMS x 4 at 4 ohms, 160 watts RMS x 4 at 2 ohms, and 320 watts RMS x 2 bridged at 4 ohms. Those are the manufacturer's figures, so expect real output to depend on your electrical supply and gain settings.

Can it drive a subwoofer?

Yes, if you bridge two channels. Bridged, a pair of channels is rated at 320 watts RMS into 4 ohms, which is enough for a modest sub. It is not a substitute for a dedicated high-power mono amp.

Does it have a built-in crossover?

Yes. It has a selectable crossover so you can send highs to speakers and lows to a bridged sub, or run all four channels full range. There is no DSP, so tuning is limited to crossover and gain.

Why choose Class D over Class AB?

Class D runs cooler and draws less current for the power it makes, so it needs less space and is easier on your alternator. Some purists prefer Class AB for sound quality, but in a car the efficiency usually matters more.

What comes in the box?

Expect the amplifier, mounting hardware and the manufacturer's documentation. Check the listing for whether a bass knob or wiring kit is included, since those are often sold separately.

Is it good for a first system?

It is a reasonable starting point if you want one amp for four speakers with the option to bridge for a sub later. Just match your speakers to its 4-ohm and 2-ohm ratings.