Awesome build, man! I love it. Can't wait to see the caiman in it. Gotta love those PA laws! I'm not allowed to have harmless sugar gliders and black rat snakes, but I could keep a saltwater croc in the back yard, and no one would care.
I agree with Conner- a simple sump is probably your best bet. It'll be the cheapest, and easiest to customize later (when you make the one larger). If you do a canister setup you'll either be spending a ton of money now, and over-filtering until you make the larger pond, or you'll spend even more money replacing whatever system you go with now.
How are you planning on heating the enclosure? Will the water be heated, as well? (if so, another +1 for the sump- you can put heaters in it) I'm sorry if I missed that earlier in the thread.