Ben, thank you so much for the detailed reply. It's very helpful and confirmed most of my research. I am going for an AO Smith B2980 pump motor on the existing pump. I'll let you know how it goes.

The only remaining question is about the main drain. Do you think I should open up the plug and look into it further (what would the next steps be?) or just let it be and keep it plugged?

Thanks,
Stefan