I agree with your assessment of the issues. The shortened life probably won't be significant. I doubt the extra noise will be significant either, but that is pretty subjective so you might find you don't like it.
If you can live with the reduced air movement, a higher voltage fan (24v) would run slower and last longer, but I don't know about availability.
You won't need a very good dc-dc converter (tight regulation, low ripple output) to run fans so an inexpensive one would do and give some control over the speed if you get one with a bit of adjustment.