Looks like the sale will go through after all, I'm getting excited...
Current owners have mainly been "floaters" rather than "boaters" so no solar and are plugged into shoreline 24/7. Batteries are listed as 1x120Ah for engine starter, 620Ah domestic, 240Ah bow thruster.
I've had a go at working out a kind of worst case energy consumption scenario, which would certainly require running the engine once a day.
https://docs.google.com/spreadsheets/d/1KtzkGQWkirfYjsbP8xQjs84RDI3BEMSjAWEjUjS4c7w/edit?usp=sharing