I've been thinking of how to do a fan wall with computer fans - I need 250+cfm (factory spec) on the inside, and 500-800 cfm for the outdoor half. I think those numbers are going to be difficult to reach with computer fans.

The heat strips would be controlled by a relay, and controlled by the thermostat. they will be locked out most of the time.

the other option that would allow me to retain the factory fan in its entirety would be shutters between the fan blades and outdoor coil. I'm not sure how i would make an off the shelf unit fit - they're usually made in standard ductwork sizes. I see no downside to letting the fan freewheel for a few minutes at a time.

It's pretty easy to find 120mm fans in the 100 CFM range. (Delta is one common brand.) A few are even into the 200 CFM range. How much area do you have to work with? The little 5000 BTU unit I hacked long ago had enough area to fit 6 120mm fans, but I only put 2 because that's enough.

I like the nidec servo D1225C12B5AP and the rest of the lineup. great bearings, good static pressure, very tolerable noise. I wasn't thinking down that route but it's worth a thought or two. only problem with computer fans is they're not aimed at outdoor use (exposed windings/drivers), but they would be reasonably well protected.

the radiator measures approximately 18" x 14".

I've been thinking on a method of control for the second fan, and I think perhaps a damper is a better idea. I found a place that makes custom sized aluminum zone dampers, which means i wouldn't have to mess with the fan at all. that would things a lot simpler, and it may help defrost the coil faster on windy days (not sure what impact the fan running behind the coil might still have).