If you must have a DC powered fan I'd be scouring eBay for RV style exhaust fans - Fan-Tastic, Maxxair & Ventline (in decreasing price ranges) have some advanced fans with tons of options - thermostatic control, rain sensors to close the dome, etc. I nabbed a take-off, a new fan an RV buyer wanted upgraded "Ventline 2501" 12V 0.5 - 5amp reversible model for for $50 delivered.
Would you consider trying passive first? Mechanical thermostatic controlled skylight or gable 'window' vent using the below actuator or similar? Remember, to get the best chimney effect you'll need at least two openings, one low and one high...
Do you know whether the fan you're thinking of is a pull or a push fan?
A radiator fan is designed around pushing/pulling air through a multi-layered radiator - if allowed to run unloaded in free-air I'm thinking its outside its optimized blade profile design, plus its not a continuous duty motor. Really - the motor bushings are made for a couple hours usage weekly for summer season only. And if its an after market replacement, not OEM, all bets are off on how long until the motor burns the brushes or bearings out.