Is this grinder good enough for cutting thick steel?

For occasional cuts on thin sheet, rebar and bolts, yes. For continuous cutting of thick plate or pipe, a corded grinder holds speed better and will finish faster.

How long do the two 4.0Ah batteries last?

Expect short bursts of cutting or grinding per charge, longer on light deburring. Heavy continuous work drains a pack quickly, which is why the second battery matters.

What discs does it take?

It uses standard 4-1/2 inch (115mm) discs with a 7/8 inch arbor, the most common size, so cutting and grinding wheels are easy to find.

Does it come with a charger and case?

The kit includes two 4.0Ah batteries; check the box contents for the charger and any case, since bundle contents vary by listing.

Can I use it for sanding and polishing?

Yes, with flap discs or sanding pads on the same arbor. Keep the speed in mind: 12,000 RPM is fast for some polishing pads, so use accessories rated for it.

Is it better than a corded grinder for home use?

It wins on portability and setup time. A corded grinder wins on sustained power and never needs charging, so pick based on where and how often you work.