What Apple Watch should I buy in order to get the ecg app?

Hi, the ECG app is available only for Apple Watch series 4, 5, 6 and 7.

On the link below you can find everything you need to know about the ECG app on Apple Watch.

Take an ECG with the ECG app on Apple Watch - Apple Support

apple watch 4 and newer

Apple Watch - Compare Models - Apple

not the SE version but mind you ECG is not avaible in all countries

watchOS - Feature Availability - Apple