You are not starting too soon. Your best bet is with Alaska Airlines. They now fly non-stop from SEA to MCO. Their prices seem a little high, but for non-stop it would be worth it. From Portland we have paid around $225.00 per ticket for the last couple of years. October should run you about the same and early December. However, if it is Christmas I would think they would run more.
Call Alaska Airlines and ask if you were to purchase tickets and the fare went down in price would they refund you the difference or could you receive travel vouchers for future travel. If the answer is yes I would purchase when you felt you had a price you could live with. Then if the price lowers that is just an extra bonus.
When Southwest opens their seats for October all the airlines tend to follow suit. We already purchased tickets for December, but we are doing a multi-city vacation and those run higher anyway.