We sailed on Explorer last Jan. to the E. Carib. and liked it so much we are sailing her again to the W. Carib. Jan. 25. It is a fabulous ship. I would sail Voyager class ships whenever possible. You will not be bored. The food, service and staff are wonderful.
As for the best time to buy tickets, many prefer to do it well ahead of time. We have booked ours a few weeks ahead, due to trying to fit the cruise into our schedule and this year waited a bit longer for prices to go down. If you want to go next month, book now. Sale prices come out 4-6 weeks in advance. We just booked at $300 less than the price our TA quoted in Sept. Less than we paid last year, too.
Go for it and as the RCI slogan says--"Get out there!"