there are a few brides that are planning/have had their ceremonies in MK. have a look in the planning journals section. one of the brides is called michelle and had a beautiful vow renewal there!
the MK ceremonies take place on the swan boat landing, down to the right of the castle. the fee is $10,000, that is not included in the wishes $12,000 minimum [as far as i'm aware]
the $10k package includes the ceremony for up to 50 guests [inc bride and groom], chiavari chairs, a string quartet and transport for your guests up mainstreet.
the ceremony is usually at 8am, before the park opens. it can be very hard to get a date [think it is confirmed around 4-6 months out] as it has to be cleared with disney that nothing else will be going on in the park that morning.
this is all from memory and may not be exactly correct!
i'm not sure about favourable dates, but i know that sometimes in the summer months Magic Kingdom can open at 8am, which probably means there won't be a ceremony on these days!