I have almost a carbon copy of the setup that you are inquiring
about.... 50G D.A.S. tank with filter box pulled out and a Fluval 304
running a RFUGF... I am exceptionally pleased with the way it has
performed thus far...9 months and thriving. It's heavily planted with
a mix of Seachem Flourite, CaribSea Eco-complete, and about a half
gallon of hydroponic-grade crushed lava rock as a substrate. Stocked to
full capacity with fish, a DIY 1 gallon CO2 canister injecting CO2
directly into filter intake. The only thing that I've done a bit
different is that I took the original polyester felt from the discarded
D.A.S. filter spool and tailored it to fit over the UGF plate to
prevent miniscule particles of the substrate from falling between the
spaces in the plate (the Eco-complete has some very fine particles in
the mix). This tank also has about 120 watts of light on it (4) 30
watt, 36" full-spectrum t-8 bulbs). I've never once had an ammonia or
nitrite problem and under 20ppm of nitrate at all times (regardless of
my occasional lapse of monthly 1/3 water change). I've known some
other folks who've had similar systems set up for over half a decade
with a very lax/poor maintenance schedule, and they've gone without a
single problem. So, in short, I'd HIGHLY recommend this method. One
concern to consider is that this provides for no surface agitation-
ideal for a lightly to moderately stocked aquarium, but if you intend
to stock your fish to near full capacity, it would be wise to put an
extra powerhead or something like that in the tank to aide in
aeration/circulation.