rooftop vs. split system in Phoenix

Recently moved into a 1976 home with an original Goettl rooftop unit. Given its age and inefficiency, we're starting to think about replacing the unit. Would most appreciate any input on whether to install a split system or replace the roof mounted system.
Home is 1800 square feet, and we are probably going to enclose the back porch which is another 200 sq. ft.
We have a flat roof, so there is no attic or crawlspace - the existing ductwork is dropped from the ceiling and enclosed.
In my completely non-professional opinion, it wouldn't be too bad to install a split system as there is a storage room in the garage for the air handler that is close to the existing ductwork (about 6 feet), and the secondary unit could be placed on the outside wall of the storage room.
Thoughts on consideration of cost, efficiency, anything else we should consider? (I have no idea of cost differential between the roof vs. split)
Thank you very much!
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You are very welcome.
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your house is plumbed ducted & wired for a package unit. put a replacement goettl in there and be done with it, or be stupid and completely redo it with a split system, spending lotsa $$$$ trying to save a nickel.
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gofish,
Thanks for your opinion! I have never done this before, and would really appreciate a bit more detail. Have searched the boards, but have not seen any 'basic' comparisons between split vs. roof - and just want a little more information before the contractors come out in case someone tries to sell me something that is not the best for this situation.
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wrote:

I swapped a roof unit for a spli here in Florida. One thing you have to factor in is fixing the roof when you pull that unit off.
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