Go for it
I've a Rena Classic 150, which is 525 litres (138 US gallons), set up as mostly El Natural. The only difference is that I'm not using sunlight but T5 lighting (the tank would be too close to a radiator if it was near the window).
It's been planted for nearly 4 weeks now, although it's been running for about a month longer than that unplanted and fishless.
For water flow I'm using three of these hydor EKIP 300W heater/pumps
Just two of these keep the water heated and circulating in an empty tank. I've just added an extra one because the plants now block the flow a bit, and I think it's good insurance policy.
I'm just letting the water flow & plants filter the water, no carbon or sponges are used - although I did carbon filter the water as it went into the tank. I've no worries over water being pumped out of the tank either.
Water parameters are: 0 ammonia, 0 nitrite, & 20ppm Nitrate
Someone else on this forum (here)
is using the next size up Rena tank as El Natural to excellent effect also.