Is 320 ft-lb enough for lug nuts?

Yes for most passenger cars and light trucks, whose lug nuts are usually tightened to 80–150 ft-lb. Heavily rusted or heavy-duty truck fasteners may need more torque or a breaker bar first.

Does it come with a battery and charger?

That depends on the kit version sold. Confirm whether the listing is tool-only or includes a 21V battery and charger before buying, since the price difference is large.

What size sockets does it take?

It uses standard 1/2-inch drive impact sockets. Use impact-rated sockets, not chrome hand sockets, which can crack under impact load.

Is a brushless impact wrench worth it over a brushed one?

Brushless motors are more efficient, run cooler and generally last longer, with no brushes to replace. For regular use the extra cost is usually justified; for rare use a brushed tool can be fine.

When is the best time to buy?

No price history is available for this listing, so there is no reliable cheapest period to point to. Compare against other 21V brushless impact wrenches before deciding.