Viessmann 100 W regular, on open vent. Chosen for simplicity, because other parts of heating system readily available, if not in van, and virtually FOC.
13Kw would have done heating but chose 16Kw to avoid extreme ends of product range,also brings benefit of faster reheat DHW.
Were my daughters still at home I would size boiler at 16Kw plus 3Kw for every female in household and increase size of DHW cylinder to match.