Its a good idea to get the appointment now. Even if the medical is not booked up it can still take a substantial amount of time as it goes through to CASA medical. Unfortunately I have been around a lot of people in the industry who have been delayed due to the time it takes to process medicals licences and ASIC's.
Have you already applied for your ASIC? have they asked you to? as that is the slowest one of the bunch.