Makes no sense to not be able to perform a manual J on any pre-existing home or building. Heck you can purchase the software and do it yourself granted you'd need to know what kind of insulation is in your walls, in your atttic, single pane or double pane windows, which way your home faces and the windows location as well as any trees and/or awnings, etc.
Go with the guy who says it CAN be done as the other guy is just trying to get out of it easy and may be the less experienced of the two.
I just recently installed a Ruud 410-A system and I went over to the customer's home today for a drainage problem (internal plumbing backing up the main drain and tripping the evap float switch so not my fault, you ac guys!

) and it is running beautifully. The owner said that last year this time her elecricity bill was almost $400. This last month it was $202.
Ruud is pretty good as well but if I were to purchase a home and have my own choice of a/c systems to install, it'd be American Standard which is Trane, like GMC and Chevy are one and the same. I just like the solid look of American Standard versus Trane.
And no variable speed, two stage cooling/compressors this and that either, just single stage. On and off, simple. X-13 motor, high efficiency in the furnace.