If you won't lose any house air heat by leaving fans on all the time that might be ok.
Kinda in other words you don't want to be keeping the exhaust side of a heat exhanger warmer than it otherwise would be when the furnace is off and cause the extra heat to be lost up a flue.
I like computer fans. Cheap, kinda quite, low power, easy to mount, etc. You can get fancier ones that have a temp sensor on them, hotter the air is the faster they run. Cooler the air, slower they run. Might be good for something like this on the hot side or in the heat ducts.
Each fan would not blow as much air (nor use as much power) as a car blower maybe. But sounds like your wanting a steady low airflow perhaps, not raging winds, so perhaps they would be good for this use.