https://agenda.com fit the bill for me. Offline use (though can only sync today through iCloud and Dropbox, I think, which sucks - but you can sync folders manually), pulls calendar from your system integration, simple payment. You can link your daily agenda items/notes to calendar invites.
Though it does not connect to email, at least today, which was OK for my use cases.
1. Completely offline use 2. No separate accounts 3. Stable interface 4. Straightforward payment model
I'm really looking for a next-generation Thunderbird, not some weird ML or smart task aggregator.
Honestly, just make iLife 2.0 while your at it. Just don't hold my hardware hostage this time please.