Why 0%
The EU–South Korea free trade agreement removes duty on cars of Korean origin. It isn't automatic: the invoice needs a statement on origin and, above €6,000, an approved exporter number.
When it's 10%
Without proof of origin, or if the car wasn't built in Korea (e.g. a European brand bought in Korea), customs charges the standard 10%.
Excise and VAT
Excise on the customs value plus duty at your engine's rate, 23% VAT when cleared in Poland. Many Korean cars are hybrids: 1.55% up to 2000 cc.
Registration
Inspection, OC, sworn translation of Korean documents, the registration office, plates.