Is this impact driver strong enough for deck screws?

For short deck screws into softwood it can work, but long or thick screws will stall. A 18V driver is the safer choice for decking and lag bolts.

Does it come with a battery and charger?

That depends on the kit you pick. Check the box contents, since some listings are tool-only and need a separate 12V battery and charger.

How is it different from a drill/driver?

The impact mechanism adds rapid rotational blows that drive screws with less wrist strain. It is not a substitute for a drill when you need to bore holes in wood or masonry.

It takes standard 1/4 in hex-shank driver bits, the same ones used by most impact drivers. No special chuck adapter is needed.

Is brushless worth it here?

Brushless motors run cooler and more efficiently and usually last longer than brushed ones. On a 12V tool that means more work per charge and less maintenance.