Aloha Russell,
It's not too surprising that your question went unanswered for a day. It's a big question with a very involved set of answers. There are important considerations when drilling a tank and using sumps.
No, a sump is not needed. It will help but it is
not needed. As a matter of fact, that skimmer and HOB filter are not
"needed". These tanks of the month(5/03 and 11/08) had none of those, just natural biofiltration and innovative water flow patterns.
I would be happy to offer my assistance. I currently have both types running, sump and sumpless. I'm in Murray if you would like to visit and talk reef.
Anything that can be conceived, can be achieved.
Mark