Central Air Replacement in condo - questions?

Hello all, I'm a long time condo owner but a first timer for central air questions. My condo was built in the late 80's and has a Lennox hp18-261v10p central air unit that blows both hot and cold air. The unit still works fine but I would like to start the process on finding a replacement for it since it's getting old and I feel it's probably not as efficient as a new unit. Is it possible to just replace the unit that is located outside that has the air intake on it or is that piece and the unit located inside my home directly connected and has to match? Sorry for the very basic questions but I'm very new to this. The duct work in my condo is in great shape so I think this would be ok to keep.
I wanted a few opinions on where to go next with this before I bring someone in to give me a quote. Plus, can anyone give me a basic idea on costs to replace the outside unit or is this question too general?
Thanks for reading this.
Doug D
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