sounds a lot like the algae filters used by marine aquarists. You might find that you would have to light that tank well. I don't know if all that water movement would knock out most of the free CO2 and if you would end up considering a CO2 suppliment. (Why does a tank of fast growing Vallisneria form an image in my mind?)

That plants business in the sump sounds pretty spiffy, but elaborate. It would require some work in maintanence. You may want to go with the very expensive bioballs (plastic chips, new plastic haircurlers, polysomething or other material (not organic like cotton) used as stuffing for pillows oe quilts, clean small pieces of lava rock, etc.) as a way to break down and get rid of (at least some) of the nitrogen related wastes. That set-up would be easier to change water into as well.

Do a google search on what the (macro) algae filter people are doing with marine set-ups. I would also seriously look around for trickle filter media which gives you the most bounce for your buck - whether that  buck is from Canada, the states or Oz. :)

An aquarist names Henri DeBruyn has devised a small trickle filter for little tanks. Basically he uses small pieces of large pored lava rock in small sections of guttering. The water is raised from the tank with the energy of just the regular airpower filter lines. Several people of have used it are singing it's praises. It suggests what could be done with a larger sump.

See
http://fins.actwin.com/nanf/month.200011/msg00203.html

for some details on DeBruyn's unit.

I'm sure there are a lot of other reasonably priced filter media that were not hinted at above. A search for wet-dry filter media or trickle-filter media (or whatevere they are calling it now) would probably yield a lot of good suggestions.