Drilled is definitely better. Easier to octo-proof without causing a drag on the drain.
75 gallons is great for an O. hummelincki. Bulkhead size won't matter too much as far as debri goes, it's the strainer on the intake you decide to use. I made an octo-proof overflow by using an extension on the internal overflow standpipe. To explain it... the standpipe is like an upside down "J". It drains from the curved end. Get an 8-inch length of PVC pipe, drill a ton of 1/8th inch holes in it, I'm talking hundreds of holes, put an end cap on it, and place it in the drain end of the standpipe, end cap down. The more holes it has in it the better it will drain. The very top of the standpipe should have a small hole drilled in it. If you have a power failure it will keep the standpipe from syphoning more water than your sump can handle. Just run a test on your system when you hook it up and make sure your sump is running with enough room to accomodate whatever amount of water drains from your tank after the return pump is shut off.
I like the Hydor Koralia pumps. Haven't had any problems with octopus arms getting damaged, but mine occasionally plays with the pump, yanks it off the glass. I think it finally got bored after doing that 4 or 5 times.