Selecting a FCU...
1) Run load calculations for the zone to be served by the AHU.
2) Review floor plans and designated area for FCU placement.
3) Determine FCU configuration, floor mounted, suspended, vertical, horizontal, ducted/ductless, etc.
4) Determine the cooling mechanism, Dx, CHW, GSHP, etc.
5) Go to manufacturer's internet catalogs or paper catalogues if you have them, search for units that match the requisite configuration and type.
6) Sort through performance data and select the one that best matches requisite load profile.