Hi!
I am new to mini-split systems, though somewhat familiar with HVAC.  I am replacing a 2-ton Bryant heat pump system that has a 2-1/2 ton air handler in the basement for cooling the back part of our Arizona home (just east of Scottsdale).  I am considering a couple of options: 
(1) Just replace the existing system with a 24,000 single mini-split indoor ducted unit connected to the existing duct system.  Total static pressure with the current system is 0.26 (with 2-1/2 ton air handler running at only 600 CFM)
(2) Use a 2-zone mini-split with one 12,000 indoor ducted unit connected to the existing duct system plus a 9,000 wall-mounted unit for the “office” room.  Running Manual-J, all rooms except the office total 8,764 BTUH.  The original system for this part of the house was ducted for the originally-installed 1-1/2 ton system, but the office was added-on later and the basic duct system was not changed.  Then the system was upgraded to the present 2-ton Bryant system and a duct was added to the office.  The office (252 sf) is the greatest single load in this part of the house – 10,334 BTUH per Manual-J, and it would be nice to be able to control this room separately from the others.  The reason the load is so heavy in the office is that two walls (East & North) are almost totally windows, and a third wall (West) is about 1/3 window.  Also, those walls are obviously exposed to outside ambient, but so is the ceiling and the floor (the room is ‘suspended’ over open space below it).  
Would appreciate any thoughts or input…..

Other questions:
(1) Is there any problem with running the condensate line into a bathroom sink drain using a ‘tee-d’ tailpiece?
(2) How long does a ducted mini-split air handler fan run after the condenser shuts off?
(3) Do you have to evacuate and pressure-test each zone of a multi-zone mini-split system individually, or is can you evacuate the entire system and pressure-test it at one time?

Thanks for your help – I really appreciate it!   Nick