My Lennox HS21 two speed 5-ton unit died a couple of months out of warranty.
The inside unit is (I assume) in great shape (Harmony-II 3-zone). What would be a compatible / recommended replacement?

One contractor is recommending a Trane XR14, which I'm leaning against both because I'd prefer a higher efficiency unit and a two-stage.

is installing a single-stage A/C a really bad idea?
can the existing air handler be used with the new R-410A A/C's?