First and most important, try to fully understand what sailing truly off-season can mean (normal charters begin in April). What about visiting India during the monsoon season?
On the other hand, an Ocean Star 51.2 for one week can cost then around 3,000 euros plus 1,000 for a skipper for one week. More information at the following website:
Provisioning costs are negligible compared to the cost of the charter. Particularly drinks will seem cheapest (e.g. a 0.5 L bottle of beer less than 2 euros). More embarassing you will find not having more seaside tavernas open (cost of meals: around 10 euros at maximum per person).
As far as the sailing area is concerned, I gather that having one week at your disposal you will sail the Saronic Islands, i.e. Aigina, Poros, Hydra and not beyond Spetsai, and that entails you will rather have light winds and much much motoring.
I wish you enjoy the cruise and feel free to ask specific details
PS: I do not comment on the choice of companies because at that time of year there is not much demand, the type of boat you mentioned is recently built and the success of your trip will greatly depend on the SKIPPER!