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You didn't state the size of the existing system but I'd assume it's a 2 ton. Correct me if it's larger.
2 ton is the smallest unit you can buy so a load calc will tell very little in this situation (matching BTU to the old unit). Remember, he's dealing with 800 sq ft. However- duct sizing will tell all. Since a/c works & blower works- if I was selling this job- I'd find out the TESP on the existing system before I removed anything. That will tell you if the duct system is sufficient or needs mods. So will a visual on ducts but I'm betting ducts are not very accessible if you have a flat roof. At 800 sq ft- I'm betting the smallest 2 ton unit will be oversized. Why aren't you addressing the lower floor? Perfect time to do so IMO.