Those hardly produce the same airflow my friend, apples and pears. A fan is not just a fan.
You will have to take a look at the airflow produced by the original fan. The Papst you indicate (which is the Rolls Royce under the fans and hence so expensive) produces 160 m3/hour while that 12V one does only 61 m3/hour, that is a huge difference.
But even if the airflow would be sufficient, you have to realize that a fan is not something that gives a constant load. The resistance that it gets from the surrounding air varies, and thus the current drawn also varies. This makes the 7812 have to work hard to keep outputting a stable 12V for the electronics that uses it.
Also, just to stick to your example, the average current used by that fan is 0,35A. The 7812 can handle a maximum current of 1,5 A. So using that fan you will only have 1, 15A AVERAGE left for the electronics. If the electronics alone draw something like 1,25A the 7812 is getting into trouble keeping up the voltage at 12V, you may even fry it over the long run.
It is just not a good idea to run fans from the power source that is also used for the PCB electronics.
So your suggestion for a separate PS would be better, but these cost money too...
There are much cheaper 230VAC fans while still a good brand like Sunon.
http://nl.rs-online.com/web/p/axial-fans/5440302/plus....you as an IT guy should be able to get some of old server racks
I scored a couple on our yearly flea-market for close to nothing.
About dB value: if the fan is too noisy you either
1) did not turn up the game sounds loud enough (like my SW cockpit)
2) did not turn the 80s music loud enough
