Is a 30 inch suitcase too big to check?

No. At 30 inches it is a large checked bag, not a carry-on. Check your airline's linear size limit, since some carriers cap checked bags near 62 linear inches.

How does aluminum compare with polycarbonate?

Aluminum is stronger against crushing and holds its shape, but it is heavier and dents instead of flexing. Polycarbonate is lighter and springs back from knocks.

Does the TSA lock work in the US?

Yes. A TSA-recognized lock lets US security open and relock the bag without cutting it. Keep the combination somewhere separate from the case.

Are the wheels replaceable?

Spinner wheels on hardshell cases are usually riveted or screwed in. Ask the maker whether replacement wheels are sold before you rely on the bag for years.

What should I check before buying?

Confirm the empty weight, the exterior dimensions including wheels, and the shell thickness. Those three decide whether it fits your airline and your lifting limits.