What other factors should I be looking at?
I'm not familiar with eyefinity setups so I can't give you personal experience. I believe the BenQ you list could run eyefinity. My only advice would be to find out what connectivity you need from the GPU to the monitors and make sure you have everything before you pull the trigger.
On a whim, I tried to run 2x 28" monitors with SLI a few months back. Long story short, I didn't do enough research and wound up unable to do it because the I/O connections on the monitors were not compatible with SLI requirements. NOTE: this should not be a problem for you since the BenQs have adequate input options.
Hope this helps.