A little extra filtration wouldn't hurt, but I'm not sure it would be essential either. I've had an experience very similar to Powchekny's. A heavily planted tank seems to do a fine job of chemically filtering your water for you. You really want canisters for water movement and mechanical filtration. But then, I'm hardly an expert.
I'd also question your plan to get juveniles. Not the saving that you might expect. Hormone production / growth rates aside for a moment, they just are much weaker than the adults - like all fish. I'd probably grit my teeth and buy some nice-looking adults.
I think your concern should be the colour, not really the size, of the fish. Fish raised with frequent, large water changes tend to have much brighter, sharper colours. But if raised properly, they should retain those colours in adulthood even with much smaller water changes - at least this seems to be the experience of the importer down the road, who has some amazing adult discus that have lived in his care on weekly 15% water changes (if they're lucky) for years.
I'd also suggest you stock more lightly, at least initially. Sure, it's a big tank, but if you want zero water changes you just can't keep many fish.
I'd suggest growing duckweed. With or without CO2, it grows crazily and really soaks up nitrates and phosphates. Of course it would require weekly removal - and if you wanted weekly maintenance I guess you'd just change the water
I'm not sure a phosphate absorber would be necessary with very heavy planting.
The important question (though I might be doubling up posts a little here) is:
Do plants absorb discus hormones? Does carbon? Does anyone actually have any proof of any of this?