I've had a couple days "away from the computer," and I've had a little time to think about this project.
First, I'm no longer 100% sure we should try to upgrade this with an overflow and a sump. Here's why: As a school student tank, this is going to be subject to some periods when it won't be easily maintained. Summertime is a big LOOOG period where the tank maintenance will be spotty at best -- realistically it won't be visited more than once a week at that point. HOB overflows are great if you're watching and maintaining them diligently, but they're REALLY notorious for failing at the most inopportune moments. Also, the evap potential from a sump (and from the opened top that we're planning) will cause roughly 1/2 to 1g per day of evaporation. Not a good idea if we're relying on a sump for the filter.
Instead, what do you think of keeping the current canister filter and using a HOB fuge for Chaeto and Pod growth?? We don't need a name brand one; there is an "off market" model that can be found easily for less than $100....what do you think?