If the cavity walls are cracked and decayed i don't understand where is the crown be supported? In this cases usually an inlay would be ideal but you will have to get a root canal done and usually root canals are quite expensive.
Have you considered an implant? Try to get a second opinion. Also remember that if the healthy tooth left (once the old filling and the decay is removed ) is not more then 1/3 of the whole crown and you decide to go with a replacement of the filling, the tooth will be very fragile.
Good luck