It seems to me that your problem is not a fan, but a poorly thought out power supply. I would, instead of thinking of fans replace the hot devices with devices from Power Trends (a subsidary of TI). I am using thousands of the PT6302 (24 to 5V @ 3A, no forced cooling), another engineer tried to make the equivalent from scratch here and came up short.

The problems with a fanned unit is dust (and you are talking about a medicfal device!) size, and noise.

Erik