You should buy it online ASAP, as usually ticket prices go up nearer to departure date as the flight gets fuller.
As for which domestic airline to take, it depends on which is the cheapest. Do cater about 3 hours for immigration and customs at JFK, then re-check in and security before your domestic flight. Also, take note that most domestic airlines charge for checked baggage, while some minority do not, so take these into consideration when booking. It would also be best, if you can interline your baggage, meaning your bags are checked all the way to MCO, so at least you avoid the JFK - MCO baggage charge. This way, you still collect your bags at JFK, but after customs, go towards the direction for passengers with domestic flights, and you can drop your bags there (instead of lugging all the way to the correct domestic terminal and re-check in again). This should also mean you should have your onward boarding pass, so you do not have to go to check in counter again, just proceed straight through security.
If you buy now, should be much cheaper than at airport (usually with additional ticketing fee). Plus last minute fares are usually full fare (unless plane is super empty). If you miss the flight, you would probably have to buy another ticket, or top up the full fare (as you would have paid at ticketing counter), plus some change fee or no show fee.
