Should I buy a spark-plug-only set or a combined coil-and-plug kit?

Buy plugs alone unless your coils are actually failing. The combined coil-and-plug kits in this pool have far fewer reviews than standalone NGK plug sets, so the consensus signal behind them is weaker. If you do need coils, verify the brand and warranty, because reviewers tend to scrutinize bundled aftermarket coils more than the plugs.