Equipment Selection & Review Level
Arc8 has provided great guidance.
40-year old firm should have Test Report as a standard.
Ask to see their Start-Up Test Report Form!
You can always hire a 3rd party to perform Test & Balance (T&B) ... however,
this additional review may be considered " a bit much of an extra" for some residential customers. However, T&B is required in nearly all Commercial applications.
TRANE XL16i should only cost a few hundred $ more,
if you wish to pursue a 2-stage set-up that
handles off-peak loads better and somewhat improved humdity control.