The motor hp (1/2 hp - AFA vs 1 hp - AFB), the size of the blower wheels and housings, and the nozzle width (2.5" - AFA vs 3.5" - AFB).
Yes, top air inlet is available for all these models. The motor bases(s) and the junction box(es) are re-located in top air inlet models.
NO - a three phase motor may not be substituted for a single phase motor. The three phase motor is larger and requires a different motor base which is welded into the unit.
The AFA Model has baked-on enamel finish and a volume control damper. The FSA is the same model WITHOUT the volume control damper. The ECA is the same model WITHOUT baked-on enamel finish and WITHOUT a volume control damper.