Why were my school projects never this cool?
Given the circumstances, I can see where price is an issue, but also given the fish, efficient skimming will be a necessity.
Honestly the one I'd consider is the MRC MR-1. It'll set you back about $275, which is probably about $100 higher than the others.
BUT, if you can possibly swing it, it's worth it, and you will absolutely be able to sell the skimmer on that famous bidding site or craig's site and get back most of your money when your project is over. MRC skimmers are coveted so you won't have any trouble.
By the same token, look on those sites for used MRC units.
Another option you may consider is to see if you have a local reef club. Reef club members are usually pretty hard_core (read: they have lots of extra stuff lying around) and love to see education about the hobby being done. If you joined the club for a nominal fee, you'll probably find someone that will lend you a nice skimmer for nothing to use for your project.
If none of these appeal to you, I'll reluctantly point you towards the coralife super skimmer. I'm not a fan of these skimmers because they tend to have problems with microbubbles in their effluent, but for a huge tank full of Tilapia, I doubt it will be a big issue.